First - it is useless because the belief of your opponent says nothing whatsoever about the state of the universe. If your opponent argues that the Earth is round, but truly believes that the Earth is flat (and maybe behaves in some external fashion consistent with that belief so that you can identify it), pointing out his hypocrisy on this issue does not make the Earth flat. Your opponent may be silly, but there is no need to join him, simply argue the merits of your position or the lack of the merits of his stated position.

Second - it can be harmful because there is such a temptation to find hypocrisy that we see it sometimes where it does not quite exist. Then, you make the big promise - "AHA - he is being a hypocrite" - and then when your audience does a little research they find that you didn't quite give the whole story. Now, you have lost credibility with your audience. And for what? The hypocrisy of your opponent would have been irrelevant to the argument in any case.

Third - it can be harmful because pointing out the hypocrisy of your opponent gives your opponent credibility.  The hypocrisy (H) is the action or belief that is not the argument of your opponent.  By attacking the hypocrisy, you are saying that his argument is weaker with H, which necessarily means that it is stronger without H.  Now you have set your opponent up as an authority figure on the matter, who needs to put his house in order, but that when he does so (or even admits that he needs to) now he has an argument that you might support.  Further, you alienate his supporters because your hypocrisy argument is necessarily personal, not about the issue, and therefore you suffer incremental loss of potential future support for your position.

The best way to treat hypocrisy is to ignore it completely. If it's that blatant, someone else will point it out, and you don't promote your opponent as an authority figure, run the risk of losing credibility for a not-quite on point accusation, or make a personal attack that alienates his supporters.

Here is how the argument by label works: 1) my opponent position is X, 2) X is a feature of [really bad label - Y], 3) suggesting (often not stated) that opponent has all the bad features of Y (or at least some other bad feature of interest).

Common labels are fascist, racist, sexist, liberal, right-wing, etc. etc.  Among the flaws here, the main one is that the feature Y is not proven to be within the opponent position just because of X. For example, Hitler breathed as we all breathe, so we all share a feature X with Hitler, but it's absurd to label everyone Hitler on that basis. Another flaw is that, even where some correlation between X and Y is accepted (i.e. the presence of X makes the bad features of Y more likely), the addition of the label makes further productive discussion unlikely.  Better is to merely discuss why, when X is present, the bad features of Y are likely. If that position is unsupportable, the argument is weak and should be abandoned anyway.

Identifying the Label is pretty easy, dealing with it is often difficult primarily due to emotional issues. Some disarming tactics include asking why the label matters, or asking what other bad features Y the labeler is intending to attach by using the label. Then, the labeler is forced to either make more specific propositions that will often be unsupported by the evidence or abandon the label.

This misunderstood creature is often mis-characterized (but not as often as "irony"), and that's kind of the point. The idea is to describe your opponents argument for him in weak or absurd terms, and provide your reader with your argument as the only alternative.  Three flaws among many in this line: distorting the argument of the opposition, omitting arguments of the opposition, and presenting a dichotomy where in reality more than two choices are actually possible. This bad argument sucks in the lazy listener because the speaker sounds like he is deliberate and is meeting the opposition head on.

This type of bad argument is easily diagnosed by recognizing when the speaker is speaking for a third party. Usually, identification is sufficient treatment, but you can also categorize the specific type of flaw if that information is readily available.