I'm just pointing out the difference between arguments in the Platonic sense and real world arguments. Credibility is never an issue in a logical argument, but there are very few real world arguments in which credibility isn't an issue. A number of the logical fallacies, like ad hominem and tu coque go straight to credibility.

That's not a reason to ignore the argument of a hypocrite. It is a reason not to ignore credibility as an important factor because of a misguided application of fallacies.