I know someone mentioned Pearl Jam earlier too as an example of a band that busts out rarities and the crowd goes nuts... I'm a pretty big PJ fan and have seen them several times, and I think that overall PJ and U2 sets are actually pretty similar - a healthy dose of whatever new album they're promoting, several "war horses" (Even Flow, Better Man, Alive, and Yellow Ledbedder have been played at every show I've been to). They may not play quite as many war horses, but they are definitely there.... then they do also play some rarer stuff.... plus some Who or Neil Young or Beatles covers (not snippets - full covers), which is something U2 rarely does nowadays.
The main difference I see is that when PJ plays the rare stuff, the ALMOST ALL of the fans know it and can sing along, and that doesn't seem to be the case with U2 - which seems to make a pretty solid argument for PJ continuing to do things the way they do, and for U2 to keep those war horses in the setlist to make the majority of fans happy while throwing some bones to the die hards.