Bob Geldof (along with others) tells the story of that first Band Aid efffort in some detail on the "Out or Ireland" DVD I recently rented and watched several times:

U2's manager was skeptical of the effort at first and discouraged his guys from participating. When it became evident that everyone else was participating, U2 got on board at the last minute. Bono supposedly read through the entire lyric and told Geldof, "I'll be glad to sing any line except THAT one."

Bob responded: "No one else wanted it, either, and it's the only line left -- you have to take it." (I'm paraphrasing of course.)

Geldof says that the line was intended to be ironic or whatever, and that in the end he was delighted by Bono's reading. Whether he fully understood what was required, or was just angry at having to sing it, his delivery gave the line an appropriate "edge."

No mention in that documentary about Bono doing it poorly at fist, and leaving, and then coming back to re-record rather than allow another singer to replace him. Of course, I can see now how that could have been the case -- omitting that wrinkle doesn't contradict the rest of the story.

With all that background, Bono's appearance on the new version takes on a bit more significance.