Hypocrisy arguments often end up failing to state what you are for. They can even come off as though you agree with the conservative principle, implying that it’s a good thing.This is a great point. If we're criticizing Ayn Rand for accepting Social Security and Medicare, that could be construed as presuming that refusing these programs is the morally appropriate thing to do.
More generally, I think hypocrisy is the most misused and overused charge in political debate. Conservatives in particular have a long and storied history of using it completely improperly (Glenn Greenwald's 2006 lecture to conservative bloggers on the subject is amusing and representative of what I've seen from them). But even in many cases where the charge is logically correct, person A has advocated principle X and does not personally adhere to principle X, you are still often just making an ad-hominem attack on person A's credibility as a means to undermine support for principle X.
The most obvious examples are the climate denier hypocrisy attacks on Al Gore. How often has anyone even vaguely familiar with the climate science debate heard deniers accuse Gore of hypocrisy over either of:
1) He has a big house that uses more energy than it would if it were a little house
2) He flies in planes to give his climate talks
Firstly, in neither of these is Gore actually a hypocrite in any way I can surmise. The case for hypocrisy rests on a presumption about Gore's views, that since he is in favour of government action to address anthropogenic global warming, he must be arguing that we all live ascetic zero emission lifestyles without technology or modern comforts. I've never heard Gore make any such claims, and instead he seems to take great pains to stress how the climate problem can be solved with modest public policy changes that spur market forces toward a green energy future.
But the larger point is to discredit the message by discrediting the messenger. Even if Gore is a hypocrite, does that modify the laws of physics by which our greenhouse gas emissions are causing our atmosphere to retain more heat energy from sunlight?
The part that is often lost in hypocrisy fights is that hypocrisy doesn't make you wrong about your views. A thief who advocates against stealing is a hypocrite, but stealing is still generally wrong despite that.
I actually do think that there are circumstances where ad hominem arguments are fair game, but at least when throwing hypocrisy accusations around, one should be aware that is what they're doing. Certainly in electoral politics the character of people on the ballot does matter somewhat, so it's not wrong to point out someone is a scumbag (if in fact, they are). But their views might still be correct.