You may consider it "advertisement", but isn't it courtesy (as well as legal protection) to request permission before posting someone else's work?? After all, they have a right to control the scope and manner of the dissemination of their works. When you post their works on YouTube, you're essentially giving everyone a free copy; you may say this benefits them, but you have no real proof, and if it were true, they'd arrange to have their works posted themselves--if they desired it. It's not your call. And it's a far cry from performing someone's song to a small audience, as at a folk club, coffee house or street corner, something that might be argued should be allowed under a rational "fair use" policy. Posting on YouTube is clearly not "fair use", it's blatant copyright infringement.