Well, I'd rather say campfire and stadium tour don't mix too well.
I have heard beautiful acoustic rearrangements of songs by other artists, particularly love REM's second (Reveal era) MTV Unplugged appearance (up on Youtube, check it out) but it's really gotta be thoughtful and fine-tuned. U2 don't do that, they really take the easy way out with the acoustic stuff (past Zoo TV)
There is no real (I won't take technical limitations as an excuse in 2010) reason for Adam and Larry to back out of playing their own music. They get paid for playing it...