I was just reading Bill Flanagan's U2: At the End of the World the other day and came across this....might shed a little light on the whole argument.
Sorry for the length...just wanted to give the whole context.
"Underworld, the vast network of work areas behind and beneath the stage, is a beehive city. On Edge's right, in a bunker two steps down, sits guitar tech Dallas Schoo with a roomful of guitars, tuners, and spare parts. It is a fully functioning guitar shop. During the concert Dallas will break off a conversation to pump a wah-wah pedal so that Edge can get the effect while keeping his own feet free to move across the stage. Just outside Dallas' room, in a cubbyhole that gives him a clear view of the stage, sits Des Broadbery at an elaborate console of keyboards and computer screens. Des runs the sequencers that fill out U2's sound and make it possible to approximate the elaborate sonic effects of Achtung Baby onstage. Des has a computer file standing by with any U2 song the band might suddenly pull out of their hats, and if it needs a synth pad or second guitar, Des is ready to drop it in. When Edge is playing the solo on "Ultra Violet (Light My Way)," for example, Des is under the stage providing a sampled eight-bar guitar figure in the background.
'There's no room for human error in what I do,' Des says. 'You have to be sharp. There's an awful lot depending on what goes on in my area. What really matters when they're up there onstage is to make sure they're with me or I'm with them.'
I ask Des what he does when the band loses their place in mid-song, as a result, say, of Bono getting excited and coming in early. 'What would happen,' Des explains, 'is I let them find out where they all are and then I go ahead of them to a chorus or verse and wait there until they catch up.'
U2 first used sequencers in concert to get a handle on "Bad" from The Unforgettable Fire. By The Joshua Tree tour sequencers were beefing up eight numbers. Now it's a rare U2 song that doesn't have Des adding some sample, phrase, or backing part."
I don't know if this means that this is the way U2 have always handled the keyboard/synth/guitar parts. Maybe things have changed since the ZooTV days.
Or am I wrong to take Flanagan's word for it? I'm still learning alot about U2, so feel free to set me straight on this.