Conducting Own Case in Criminal Trials
It is often said that a Doctor should not operate himself
Similarly it is said that a Lawyer (Yes, I mean a Lawyer!) should not conduct his own case personally i.e. he should not Cross Examine, Lead evidence etc in cases where he is the accused and must allow other Lawyers to do so.
Why is it so said? What is the disadvantage or harm in conducting own Trial by a Lawyer or by any other person for that matter? Please provide reasoned and legally backed discussion