Major bands in stadiums, with in-ear monitors, and enormous stages, use click tracks to keep the band together. If their loops have tracks on them, they usually have a conductor too. This not only helps them stay together, but confirms they are tracking along in the right place with the loop. There is nothing more embarrassing to play a song, which has a violin section that comes in at 3 minutes into the song, and when you arrive at that point in the song, the violins start one measure earlier than they should. With the conductor calling out the various song sections, your sound man or drummer has the opportunity to bail on the track, before the audience hears the musical conflict.
Our music team at church use ProTools, Reason, and Ableton Live to construct and trigger our loops. We play everything to a click track. Again, it's to ensure we are staying together as a band.
I hear criticism from musicians occasionally that boast "my timing is so good, I don't NEED a click track". What I've found, however, is the complete opposite. The dudes that make that statement, actually cannot play along to a click/metronome without crashing and burning - in an embarrassing way, because they do not play consistently. They are not permitted to participate in our band, if they cannot stay tight with a click, because that is essential to what we do.
You will find, that at the pro level, even the lighting system is tied to midi commands from a loop that the band is playing along to. Just like Axe FX scenes, are pre-determined to recall the correct effect combinations, and can be programmed as a midi track in a DAW (watch Periphery rig rundown - they no longer have midi foot controllers on stage, their pro-tools loops send their patch change and scene change commands).
Journey claim that they listen to a click, but that they have no other vocal or instrument tracks in the loop. I don't know whether I believe that. Most pro bands are playing to a click, and the vast majority have instrument/vocal tracks that the audience hears to make the band sound better and provide instruments that they can't take on the road with them.
I personally LOVE the order and cleanness it provides - we are tight every time we play. And for any band, but especially a U2 band, where the delay is the backbone of the song, having a click that is locked in, and the tempo programmed in the preset, is the only way to go. It prevents the band from drifting off the tempo, and the player having to use a tap tempo to keep the delays in the pocket. Tapping the tap tempo and having the processor change the delay time to the new tempo, can cause audible artifacts that are not pleasing. A click track to lock into, avoids that ever happening to you.
That said, this morning our singer got off the click by 2 beats. The techs didn't mute the tracks that were coming in, so I'm sure it sounded like a train wreck for about 5 measures, until there was a long hold in the middle of the song, and we just "waited a little longer" until the loop caught up to us! And, after playing to a loop for the last 7 years, that has only tripped us up about 3 times (that I've been involved with anyways).
Here is a typical loop we use:
and here's what we sound like LIVE, when we play along to that:
The examples are in different keys. That is the best loop only, and then live recording demonstration I had quickly. We often transpose the songs depending on who has to sing them - different range guys, or maybe a female. But you get the idea.