I don't think I've ever walked out of a gig early - generally, if I've payed to see a band, it's because I like them, so am unlikely to walk out. I also don't get to many gigs, so I make sure I see the whole thing. Plus, I'm a tight bastard, and can't stand the waste of money.
Even when me and the missus were the only two people at a gig in Derby once, we stayed until the end.
Outnumbered by at least two to one by every band on the bill!