In the applications of weak Hadith there are 3 opinions as follows:
1. Weak Hadith is accepted as evidence for rulings or for encouraging righteous behavior.
2. The weak hadith cannot be used whatsoever.
3. Using weak hadith only as proof for encouraging good deeds but not as proof for rulings.
From this it seems that the scholars who supported these views were in fact talking about weak Hadith that have been graded hasan, but since the term hasan was only used post-Trimidhi era, the scholars didn't use the term.
If that is the case, then all three opinions could be correct since the first two are in reference to hasan hadith and the third opinion is talking about general weak hadith.
The term hasan was used before at-Tirmidhee according to its standard usage or close to it. It was used by several scholars including Maalik, Ibn-ul-Madeenee, al-Bukhaaree, Aboo Haatimar-Raazee and others. The difference was that at-Tirmidhee was the first to define clearly what he meant by this term and spread its usage.
On the other hand, the view that Ahmad or others used the term hasan to refer to inauthentic ahadeeth which are not extremely weak is, in fact, particular to Ibn Taymeeyah and some other scholars and is not shared by the majority. Ibn Taymeeyah only advanced this theory in order to negate that Ahmad would allow the usage of weak ahadeeth, and Allaah knows best.
However, all this theorizing is not really necessary since there is an obvious difference between preferring a slightly weak hadeeth over a man-made opinion (which is narrated from Ahmad) and allowing the use of weak ahadeeth in general or in certain areas (which is the issue here).
Therefore, the three opinions on the legitimacy of using inauthentic ahadeeth are mutually contradictory and only one of them can be correct, and perhaps the best proof of this is the severe disagreement between the supporters of these three views and the implications which result from them.
And Allah knows best.
 Dr. B. Philips, Usool Hadith, p 44
 Dr A Al-Hudhair, The Weak Hadith, p 291-299< Back to Questions