I have occasionally done this, for stories that are of personal importance to me. I won't argue that it's absolutely justified, but there are a few reasons I can still sleep at night after doing it:
1) I love the HN community, and would never do it for a post that I don't think is of interest (for that matter, I wouldn't submit something that I don't think would be of interest in the first place, so maybe that cancels out).
2) It's never been more than about 2 votes, beyond which it floats or sinks on its own merits. Yes, it can be argued that a story floats or sinks on its own merits to start with, but there IS an element of luck - the 'new' page moves pretty quickly now. If a story is only "worth" 20 votes in its lifetime, what are the odds that one of those votes will happen when it's sitting on the new page at 1 vote? Asking for an initial boost doesn't put a story on the front page anymore, but it does guarantee that the community passes an actual judgement on it.
3) I don't abuse it (by my own definition of 'abuse' - whose else would I use?). I don't do it more than once or twice a year, and it's never for ego, karma, traffic to my blog, etc. It's for something that is personally important to me, and potentially useful to at least some members of the community.
There IS one thing that bothers me:
1) It can too easily be a slippery slope. "Just this once" or "just a bit" can too easily turn into "well, everyone's doing it", to the obvious detriment of the site.
Overall, I see it as asking a friend for a selfish favor. If it's done too often, or without giving anything in return, or without regard for the cost to the friend, then it's a problem. If it's done judiciously - give and take - it can be the basis for a stronger relationship.