I don't have any problem with actors appearing in commercials, after all, that's what they do, they take on various roles and act them out. Adverts are often little dramas anyway. Likewise song writers sell performance rights to songs. If they're happy to have one of their compositions used for an advert, I don't see it as selling out.

I am, however, a little uneasy when people who are well known in their own right do adverts, for example politicians or news readers or documentary presenters. This is much more of an, "I'm trustworthy and I endorse this product," sort of thing and I don't really like that -- unless they genuinely do swear by product X or service Y.