Most hadith scholars had deep knowledge in hadith, why they quoted da'if narrations in their compilations then?

Question

Most hadith scholars such as imam Ahmad, abu Dawod, an-Nasa-i (who has been said to reach the level of al-Bukhari and having even reached a higher level than Muslim -see the later quot from a-Dhahabi's siyar a'alam an-Nubala'), ibn Majah and at-Tirmidhi had deep knowledge in hadith.

ولم يكن أحد في رأس الثلاث مائة أحفظ من النسائي ، هو أحذق بالحديث وعلله ورجاله من مسلم ، ومن أبي داود ، ومن أبي عيسى ، وهو جار في مضمار البخاري ، وأبي زرعة ، إلا أن فيه قليل تشيع وانحراف عن خصوم الإمام علي ، كمعاوية وعمرو ، والله يسامحه . (Source: Syiar a'laam an-Nubala' or here سير أعلام النبلاء)

This statements sets an-Nasa-i more or less equal (one could say close) to al-Bukhari and abu Zura'a in the knowledge of hadith and it's issues and 'ilm ar-Rijal and clearly sets him higher than Muslim, abu Dawood and abu 'Isa at-Tirmidhi. The only objection one may have is his tendency to set 'Ali ibn abi Talib () higher than his oponentes Mo'awiyah and 'Amr ibn al-'Aas.

Why they nevertheless quoted in their compilations da'if narrations?

Answer 1

Scholars had various reasons for including weak reports in the collections.
For example, they may have differed over a narrator and whether or not he was weak, one Hadith scholar may have known a support for a narration which raised it in his personal judgment and it's possible that a scholar didn't know a Hadith was weak due to some masking of identities in the chain. Also, scholars differed over what constitutes an authentic or good report.

Furthermore, the Scholars of Hadith differed over whether or not weak reports can be used in the absence of something authentic. Ahmad and others were strict when it came to Halal and Haram and more lenient when it came to virtues and history while Abu Daw'ud and others preferred a weak report over personal opinions provided it wasn't severely weak.

Additionally, weakness varies in strengths such that a report can be slightly, mildly or very weak and a report can be weak but the Scholars are unanimous over the authenticity of its meaning. Ibn Hajar includes some of these reports in Bulugh Al-Maram.

Finally, Scholars may have narrated a report simply to clarify its weakness, others, like As-Suyuti wrote encyclopedias of Hadith without the intention of using them to extract rulings.