A high-rep member told the OP of this question that saying 'thanks in advance' is rude. Why is this so?
I've seen this same user complaining about this before, and started an argument with 3 other users about it. I flagged the post and all the comments were deleted. (By the way, 1000 Rep is not really considered "high-rep"). Ignore him and the comment.
"Thanks in advance" is not rude in my book. I'm from the U.S. if that means anything.
Pointless? Definitely. Rude? That's quite a stretch.
If you want to, edit it out and flag the user's comment as unconstructive/obsolete, and otherwise ignore this type of comment. It's certainly not helping anything, and if the user really is offended by it, he is more than welcome to suggest an edit instead of complaining in the comments and wasting our time on meta, which IMO is rude.
In a linguistic sense, I think some people can interpret "Thanks in advance" as rude because it may indicate that you're not interested in sticking around for an answer - you're just going to come back and pick up whatever is left when you return. So I've heard, at least. On the linguistic scale, there's a better site to ask if you want to know how "Thanks in advance" can be interpreted as rude in this language.
In a site policy sense, we don't like "Thanks in advance" and similar not because it's rude, but because it's against our signature-and-salutations policy. You can read up on the basics in this question thread. In summary, it's basically a noise that's inbetween greetings (which take up space and delay the onset of reading the question) and signatures (which take up space and are extraneous), in that it takes up space as noise that doesn't contribute towards the actual question you're asking.
I don't think that it is. But there are some issues with its use.
I'll list the ones that bother me:
People keep writing "thanks in advanced" instead, and that really irritates me. I'm sorry, but I can't help it: just knowing that you think there's a "d" on the end makes me angry.
As others have indicated, "thanks in advance" implies expectation of aid. To be fair, it's a realistic expectation given the nature of this site. I can see why someone would think it "rude", though I think that's a bit of a stretch. Really, though, "please" is far more appropriate; "thanks in advance" just seems a bit obnoxious.
This is already well-covered elsewhere, but "thanks" in general can seem a bit slimey. You're not actually demonstrating gratitude in any meaningful way. Did it put you out when you wrote "thank you"? Did it cost you anything to write that? You're pretending that you're being polite and grateful, but actually all you did was type a few characters. That's a little insulting. Instead of writing "thanks", just contribute to the community by voting, answering and commenting. That's how you can actually demonstrate gratitude... not by writing a few characters and pretending that it means something: no matter your kind intent.
Saying "thanks" is a way of indicating that you have received something of value (or at least something you perceive to be of value). Thanking someone for a correct or insightful answer, for example, is certainly appropriate.
Where saying "thanks in advance" is problematic (in my opinion) is that it divorces the display of gratitude from the receipt of useful information. In other words, you're thanking unspecified people for things that they have not provided to you yet.
As noted already, it's certainly a pointless thing to say, but I suspect that it could also be interpreted as an indication that the poster does not value your time personally and is merely "going through the motions" in the hopes that it will land him an extra answer or two.
However, given the massive proliferation of language/cultural barriers and "newbie" developers (and the relative scarcity of genuine sociopaths, at least among software developers as far as I'm aware), I would tend to err on the side of, "The poster means well but is uninformed".
In advance means "ahead in time." Thanks in advance means you are thanking people who answer before you get any answer. It doesn't seem to be rude, and it doesn't seem to imply that you are not going to read the answers given for your questions.
That said, normally on SE sites it's preferred if the question just contains the question being asked, and not phrases similar to "any help is appreciated," or "thank you for reading me."
Such phrases, even if they are an extra, are surely less noisy of any phrases added before the question itself, such as "Hello guys," or "Hello Drupalers," or "Ave, conncetioniorum virtualorum habitantes!" That is probably the reason why the greetings in a question have been removed before the thanks, which (as far as I know) are not yet automatically removed.