I wrote previously about thank-you (with the hyphen, it’s noun or adjective only – not the actual expression of thanks).
Someone I follow on Twitter (@BrendanCormier) identified another problematic usage involving gratitude: thanks in advance, which he calls ‘one of the most insidiously awful phrases in the english language’.
I agree wholeheartedly with that sentiment.
Even though it’s grammatically unassailable, stylistically it’s deplorable.
Presumably your request also incudes please, so the anticipatory thanking is redundant. Thanks in advance is also becoming a shop-worn cliché (so to be avoided for that reason alone).
But more than that, the phrase suggests that the writer won’t necessarily bother to thank you when you do fulfil the request.
This is just rude and more than a bit passive-aggressive.
Please don’t use this dreadful phrase, and just thank when appropriate.