To a degree, the ridiculous number of guitars The Edge takes on the road has to do with Bono...
His vocal range simply isn't even close to what it once was, and the rigors of touring and performing 2.5-3 hour shows is a singer's worst enemy. "Most" U2 stuff was recorded with The Edge and Adam tuned a half step down. There's a sprinkling of original recordings done in standard "E" tuning, as well as a few songs (From The Unforgettable Fire) that were recorded in a lower tuning and then "sped up" to sound as if they are in standard. Both "Pride" and "Bad" are examples of that trickery. Even then, he (Bono) was pushing his range to the very limits in the studio. There are lots of clips from documentaries about the making of their records where you can clearly hear him falling well short of the mark.
Anyway, their live sets have been modified around Bono's comfort range. When I saw them last year, some of the songs were tuned down as much as 2.5 steps (Dallas mentions a C# tuning in the video). Basically, each song has been transposed to some extent in order to save Bono's voice.
I noticed immediately last year when they started playing "Sunday, Bloody Sunday" that it had been dropped WAY down. I'm not sure how Edge managed any string tension at such a tuning.
Anyway, moral of the story:
Thanks Obono!