I'd only do it if the money was right (at this point, that wouldn't take much!) but for big acts, I can't see tha the 'exposure' would make it worth the fans one could lose form doing it. then again, maybe the normal 'fan' wouldn't notice or even care?
Speaking of which ...
last night my wife dragged me to a Cheap Trick concert - yeah, they're still around.
Venue was crappy, as were the audience members, as a whole. I'm pretty sure that the lead singer, Robin Zander, was lipsynching the whole time. More than a few evidential things pointed to it. The instruments were live, as were SOME of the backing vocals.
My reverbnation site: http://www.reverbnation.com/mikebirchmusic
Youtube: http://www.youtube.com/mikebirchmusic
Facebook: http://www.facebook.com/mikebirchmusic