I do not agree with the statement that anyone who could read could claim benefit of clergy.
The basic idea of benefit of clergy was that Church people could only be tried by Church courts. This meant that a priest who had committed a crime could only be tried by the Church and could only be punished by the Church.
This has been used up until recent times in some countries. For example, one of the major disputes in Mexican history between conservatives and liberals has been whether to take away the Church's "fueros" -- it's right to be outside the law.
I am not aware of any countries where it is still used. You could argue that the current sexual abuse scandal in the Catholic Church is related to Church people trying to take an informal benefit of clergy without actually having that in the law.
No, benefit of clergy is not used, it was a medieval limit on capital punishment. Back in the 15th century when a person was convicted of a capital crime he was entitled to claim "benefit of clergy". This meant that if the convicted person could read, they could not be executed for their crime. By the end of the 18th century this concept was no longer used.
Some authors associate this with the present day concept of not executing a mentally retarded person. The theory is that even though it is true that they committed the illegal act, their minds can not differentiate between right and wrong and therefore can not form the requisite mens rea(criminal intent, state of mind). Obviously, this has been and continues to be highly debatable.