It's interesting the way different bands work. A lot of the bands I listen to are of the sort that tour all the time, whether there is a new album or not. As songs are worked up by the band, they make their way into the set. Sometimes they are played frequently. Sometimes, just once or twice. Sometimes those songs end up on the next album. Sometimes it's a few albums down the pike. And sometimes they never make it to an album. Sometimes the studio version is drastically different than the original, and sometimes it is just a polished version.
It would be cool if U2 did something like that, rather than sticking to their own formula, and a setlist that is (for the most part) static. If they were to work up tons of songs and rotate them on a nightly basis, that would not only be a welcome treat for the concertgoer and boot collector, but it would also keep the band on its toes.
My other favorite band in the world, Phish, rarely retired a song from its live show and, by the time they split up, went on the road with a list of almost 400 different songs, that could be pulled out at any time. They were also a band whose fans would slay them if they played the same song two nights in a row, or even in the same week, which would never work for U2, but it's still an interesting basis.
Of course, the two bands are of completely different musical universes (linked together only by Steve Lillywhite) and U2's formula seems to work for them, so I shall shut up now and continue salivating for the new album....