Some people who do not have good understanding criticize the scholars of Hadith concerning Jarh because they did not understand the purpose of Jarh. The scholars of Hadith spoke about Jarh (criticism of Hadith narrators) and Ta dil (validating the uprightness of Hadith narrators) as a precautionary procedure to protect the religion, guard its rulings, and determine the points of faults and mistakes that take place in this great basis [i.e., Hadith] upon which Islam and the bases of Shari ah are based.
It is impossible to think that the scholars of Hadith wanted to attack people and backbit them, but they explained the weakness of some narrators to avoid taking Hadiths from them in order to protect the religion. It is important that the witnesses of transmitting Hadith to be more trustworthy than the witnesses in their testimony to rights and properties. Therefore, they imposed upon themselves to speak about Jarh and Ta dil and explain the conditions of people, which is more useful to the religion.
Ibn Sirin said: In former days [near to the time of the Prophet peace be upon him], people did not ask about Isnad (the chain of transmission), but when seditions occurred, people started to ask about Isnad to take Hadiths from the people who follow the Sunnah and leave out the people of heresies because all the people of that time had good memories and could memorize Hadiths, but some people were not entitled to narrate Hadiths.
Each person was accused of lying in Hadith or was a fool who made many errors should not engage himself in narration according to the scholars of Hadith.
Some people of Hadith spoke about a group of grand scholars and graded them as weak narrators in regard to their memory, whereas others graded them as trustworthy because of their status and truthfulness even if they had committed some mistakes in their narrations. Do not you see that Al Hasan Al Basry and Tawus spoke about Ma bad Al Juhany, and Sa id ibn Jubayr spoke about Talq ibn Habib? Moreover, Ibrahim An-Nakh y and Amir Ash-Sha by spoke about Al Harith Al A war.
Likewise, Ayyub As-Sikhtiyany, Abdullah ibn Awn, Sulayman At-Taymy, Shu bah ibn Al Hajjaj, Sufyan Ath-Thawry, Malik ibn Anas, Al Awza y, Abdur-Rahman ibn Mahdy, Yahya ibn Sa id Al Qattan, Waki ibn Al Jarrah, Abdullah ibn Al Mubarak, and others from the scholars of Hadith and the Fiqh (Islamic jurisprudence) spoke about people and graded them as weak narrators.
People after them had followed the same track and spoke about narrators to know their conditions.
Muslims are unanimously agreed that it is obligatory to use the narrations of the truthful, the reasonable, and the good memorizers of Hadith?!
This is enough reason to criticize narrators who do not have these qualities and show their conditions to know the status of people from whom you take narrations and proofs.