In advance means "ahead in time." Thanks in advance means you are thanking (who answers) before to 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 begin 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.