I try to use somewhat obscure songs on my mixes, not to be indier-than-thou, but just because
a) it's what I listen to
b) they're often more likely to have fitting lyrics
c) I view mixes partially as a way to discover new music, and I don't want people to have to download a bunch of songs they already have

Which is to say, there's nothing wrong with mainstream artists, but reverse music snobbery is equally irritating.