Question: What is the benefit in the students of knowledge learning about the (deviant) sects of the Mu'tazilah, the Jahmiyyah and the Khawaarij, since they do not exist in our time?
Response : Learning about the deviant sects in our time has benefits, (from) which are:
That we know the basis for their (deviated) positions so that we can refute them if we find them, and in reality they do exist; And the questioner saying that they do not exist now (in our time) is based upon his knowledge only. However, that which is known to us and other than us who read about the affairs of the people is that these (deviant) sects are in existence and they are active in propogating their message. And because of this, it is imperative we study their opinions/views so that we know their falseness and know the truth (that we are upon) and refute those (of them) who discourse with us.
Shaykh Ibn 'Uthaymeen
Kitaabul-'Ilm - Page 128, Question No.25