Tu Quoque

Do you have a specific example? Granted, I am a social psychologist not a clinical one, but I have never heard of a universal claim that narcissists being described as "the biggest hypocrites." I haven't seen any data that narcissism is scientifically correlated with hypocrisy, so I would check that assumption. The goal of the psychologist is not to win an argument - or even present the most logical one; it is to help the client/patient. Calling out hypocrisy is not problematic in itself; it is problematic, as you noted, when it is used as an argument against someone's claim. Again, an example might help here.

I have to admit that Tu Quoque arguments are among my favorite pet peeves of all the other ad hominem fallacies. Mainly because I strongly suspect they are rooted in deeper psychological reactions (projection and deflection) and cognitive biases (agency detection). When confronted with an impulsive tu quoque response I immediately suspect the opponent is immature and naive rather than narcissistic or deceitful. They are stuck in a "us v them" binary memoriter rather than a disciplined method of logical ratiocination. 

I call it the Pee Wee Herman argument, " I know you are, but what am I?"