If you were a cautious band, a calculated band who played nearly the same set every night as to avoid disruptions and errant performances, would you choose a taped backing track or a live instrument when in both cases neither would be visible to the audience?
In truth, I think some of it is triggered backing tapes, and some of it might be live, depending on the song and the co-ordination of the sequencing and all that jazz. I don't think the track on All Because of You from the Brooklyn Bridge is Dallas Schoo under the stage, I think it's a tape. But do they use a live synth player for certain songs? Possible. I'd say it's parts of both. Something tells me it's easier to play to a backing track for U2, the sequencing of the lights and the video performances, that most of it is cued up. Does it matter? Of course not.
It's like the strings/organ intro to Streets. You could make a case that someone is playing them live on a synth under the stage, or that it is a backing track, the same track they use for every show.
You wouldn't really be able to differentiate the two, I lean towads backing tracks and tapes over live playing, which doesn't really make that much sense. More risk of a mess-up.
