Acting Upon Weak Hadith

In the old days, when scholarship was taken for granted (because scholastic 'alims were many and accessible then), even the public knew that a weak Hadith can form the basis of an 'amal. Imam al-Nawawi (may Allah be pleased with him!), in his popular work the Adhkar, says:

"The specialists of Hadiths [i.e., Muhaddith] and the jurists [Fuqaha'] and other (scholars) have said that one is permitted, and in fact is recommended, to use weak Hadith in matters of 'extra acts of devotion' [Fada'il] and in 'arousing one's desire to do good and inspiring one's fear from doing evil' [Targhib wa al-Tarhib]--as long as it is not a fabricated Hadith. As for the legal rulings pertaining to what is lawful and unlawful [al-Halal wa al-Haram], buying and selling, marriage and divorce, and others like it [because all of them involve either an injunctive legal ruling [Hukm Shar'i Taklifi] (such as Haram and Wajib) or a stipulatory legal ruling [Hukm Shar'i Wad'i] (such as Shart and Mani')] are concerned, one can only use a rigorously authenticated Hadith [Sahih] or a well authenticated Hadith [Hasan], except if a precautionary ruling [Ihtiyat] is [involved] in some matter relating to one of them. So, if a weak Hadith is found to object against some types of sales or some form of marriages, then it is recommended to avoid it (i.e., the sale or the marriage) even when it is not obligatory to do so [and even when the sale or the marriage is legally valid]." (al-Nawawi, Adhkar, 7-8)