What? I totally don't buy that. With a catalogue as deep as U2's, there's multiple songs they could choose to convey a particular message. To use Vertigo's themes:
Early days: 21 songs to choose from on Boy and October, plus b-sides and unreleased material (hey, if they can play Party Girl, why not Touch or Cartoon World?).
War/violence: LAPOE, SBS, Bullet, NYD, Like A Song, Exit, God Part II, Please.
Social activism/Africa: Pride, Streets, One, MLK, In God's Country, RHMT, MOTD, Peace On Earth, WILATW, Crumbs, Miss Sarajevo, RTSS, Bad, Wire.
AB/Zoo: Twelve songs from Achtung, plus b-sides, and maybe even the Zooropa stuff could fit in.
If U2 drew from just that selection, they could have an incredibly varied setlist without sacrificing their message.