Ad hominem tu quoque refers to an irrelevant accusation of hypocrisy. Accusations of hypocrisy are inadmissible in legal and scientific debate, and can be distractions from the business of politics. That is, it is not relevant to the credibility of a didactic argument whether its presenter has trod over the principle he espouses.Take that Talk Radio! Take that annoying Letters to the Editor!
Not only is this kind of criticism a baseless personal attack, but it's one of the weakest and least meaningful kinds. Hypocrisy itself is considered "Irrelevant" and "inadmissible" as a counter-argument.
Not to mention it's sort of stupid and adolescent in a Catcher In The Rye "phony"-sensitive sort of way.