Summary: Sometimes a weak hadith defense is used to defend Muhammad's bad behavior.  The hadith are the records of the oral traditions of Muhammad's sayings and actions that were recorded by ancient Muslim scholars. The hadith contain embarrassing accounts of Muhammad's behavior. Consequently, many modern Muslims claim many of these hadith are non-authentic.
1.  The Sahih Al-Bukhari and Sahih Muslim collections are considered to be authentic historical records by orthodox Muslims.  So, a weak hadith defense is  not justified when these sources are used to critique Muhammad sayings and behavior.
2.  A weak hadith technically refers to the isnad and matn. The isnad refers to the completeness of the chain of narrators and the reputation of each individual narrators within the chain of oral tradition that goes back to the eye witness of the act or saying of Muhammad. The matn refers to the text of narration. Even if there were a technical flaw in a hadith, it does not necessarily mean that the hadith is not an authentic one. Authenticity and weakness are two different concepts.
3.  The Qur'an has less technical support for its authenticity than do many hadiths.  Yet, the Qur'an is considered authentic by traditional Muslims.  So, if an historical saying must be rejected because it is technically weak, then much of the Qur'an would have to be rejected too.
4.  Some Muslims use the weak hadith defense, because they approach Muhammad with their own wishful presuppositions. So, they automatically reject any hadith that does not meet the standard of their uncritical assumptions.  Our beliefs should be grounded in historical reality: not wishful assumptions.
5.  It is illegitimate to reject a hadith on the basis of its matn. What we like or dislike is not an appropriate standard to judge past history.  Present-day beliefs don't determine the events of the past. 

There is extensive historical information on the life and teachings of Muhammad that is found in the written collections of the ancient Muslim oral traditions. These written traditions are called hadithss¹. Some of the most valued collections of hadith were those collected by Bukhari, Dawud, and Muslim, who were ancient traditional Muslim scholars.²

Their extensive collections provide detailed information on the thoughts and actions of Muhammad. These collections must be studied in order to understand Muhammad’s life and teachings. In fact, these collections provide vastly more information on the life and teachings of Muhammad than does the Qur’an itself.

However, when someone examines these accepted hadiths, they find some accounts in which Muhammad displays unfavorable behavior. This behavior is an embarrassment to many Western Muslims who seek to defend Muhammadanism in the Western world. Often, these Muslims seek to minimize these historical accounts by claiming that these hadiths are weak; and, therefore, they should be ignored in discussion of Muhammad’s religion.

But, is this weak hadith argument a legitimate defense of Muhammad’s character? Or, is it a convenient ploy to divert the attention away from the unfavorable actions of Muhammad? Why should the ancient writings of the great traditional Muslim scholars be set aside so conveniently whenever Muhammad’s questionable behavior is presented from scholarly traditional Muslim sources by non-Muslims?

Now, it is true there were many hadiths that were rejected by ancient traditional Muslim scholars. They rejected them because these traditions were fabricated (maudu’) for political reasons long after Muhammad died. So, it is proper that these fabricated traditions should be rejected by those who follow the life and teachings of Muhammad as well as by Western scholarship. These fabricated Muslim traditions are very much like the apocryphal post-New Testament writings that appeared long after the genuine New Testament gospels were written.  Now, it is clear that Al-Bukhari, Imam Muslim and others did not intend to include maudu' hadiths within their collections.  In fact, they applied stringent standards to assure that these mandu' hadiths were excluded.  

For an oral tradition to be accepted as sahih (sound), it must meet important requirements. And, if a tradition did not meet the requirements for a sahih hadith, it was not accepted into the family of sahih hadiths. However, the standard hadith collections are considered by traditional Muslim scholarship to be sahih. Therefore, it is unjustified to use a weak hadith defense when a sahih hadith is used to present the actions and teachings of Muhammad.

Literally means sahih means sound, healthy, and without fault. Firstly, to be a sahih hadith, the hadith must have an impeccable chain of transmission (isnad) in which there is no weakness. Each link of the chain must be connected by a narrator who heard the narration from the prior link in the chain.  The chain of narrators must be an unbroken chain.  Secondly, each individual narrator must be a just (‘adl) Muslim of good reputation. Thirdly, the text (matn) of the hadith must be in accordance with orthodox Islamic teachings.  

When a hadith meets all the proper qualifications for a sahih hadith, it must be accepted by all Muslims. A sahih hadith is an obligatory hadith. It must be acted upon according to the consensus of the Muslim scholarship. The sahih hadiths are those used as the sources of Islamic jurisprudence (Usul al-Fiqh). And, it is proof in cases involving Islamic shari’ah law.

So, it is not legitimate for a Muslim who professes to follow the teachings and behavior of Muhammad to downplay sahih hadiths. These sahih hadiths are not weak hadiths in any technical sense.

Furthermore, the fact that a hadith is not a sahih hadith does not mean that the hadith is not a true report regarding Muhammad’s life. It simply means that, technically speaking, the isnad of the hadith lacks the high standard required to be a sahih hadith. So, even though a hadith is technically weak, i.e., its isnad is not flawless, it may still be an authentic hadith. Technically, a weak hadith is not the same thing as a ‘forged’ (Maudu’) or fabricated hadith. 

Fabricated hadiths were not intentionally included in either Bukhari's or Muslim’s collections. So, simply setting aside a hadith as weak is only a statement regarding its isnad. It is not a statement that the traditions is a forged or a Maudu’ hadith.  The fact that a hadith was included in Bukhari's or Muslim's collections is strong evidence that the hadith is an authentic one, even though its isnad may not meet the standard required to be a sahih hadith.  So, the weak hadith defense against an unfavorable hadith regarding Muhammad's behavior is really a weak intellectual defense.  What a Muslim must show is that the hadith is not historically authentic and this is a much more difficult task to do.

Furthermore, Muslims accept the text of the Qur'an without hesitation.  However, each ayah (verse) of the Qur'an is not based upon Mutawatir (multiple chains of corroborating narration continuous through history) isnad.  So, why should a hadith have to meet a higher standard of historical verification than the Qur'an itself?

Lastly, some Muslims disparage some hadiths because of their initial presuppositions regarding the sayings and behavior of Muhammad.  For example, they uncritically assume that Muhammad was a perfect and flawless individual whose sayings and actions were all divinely inspired by Allah.  Beginning with this initial presupposition, they set aside everything in traditional Muslim scholarship that does not accord with their initial religious assumptions.  However, this is unfortunate, because historical reality should take precedent over our religious assumptions.  Instead, our religious belief should be in accordance with historical reality.  This is why the historical record found in the ahadith and the Sirat Rasul Allah by ibn Ishaq are so important.

Finally, the third standard (matn) that Muslim scholarship uses to judge the authenticity of a hadith seems to be an illegitimate standard.  The real standard should be the historicity of a narrative.  To determine the actuality of an historical event, the chain of narrators (isnad) and the reliability and reputation of the narrators ('adl) are valid considerations.  By contrast, it is not intellectually appropriate to reject a historical event because it does not agree with Muslim religious doctrine.  This view gets the cart before the horse.  In essence, it asserts that, even if a historical event actually happened, Muslim scholarship would reckon that it did not happen because it conflicts with their present-day religious dogma.  Our present-day thoughts don't create the events of past history.  If they did, there would be no point in historical research.  

As a side note, some Muslim scholarship applies their third standard to the events of the Bible.  For example, some believe that all prophets lived sinless lives.  Since Muslim belief teaches that adultery is a sin, it would follows that no prophet committed adultery.  Therefore, since they believe that King Dawud (King David) was a prophet, it follows that he never committed adultery with Bathsheba (2 Samuel 11:2-5).  Therefore, they conclude that the biblical report of King Dawud's adultery is not a true report.  For them, the historicity of the report is irrelevant.  It does not make any difference whether or not King Dawud actually committed adultery.  Their present-day beliefs take precedence over the actual events of history.  Thus, there could never be an historical event that could conflict with Muslim dogma.  Such an arbitrary standard safely protects the Muslim's religion from the realities of the historical record, making their religion non-falsifiable.

For some additional information on the classification of hadiths, see the Introduction to Sahih Muslim's collection on page ix.