I go both ways on the piping in, to be honest. I think it depends on the band and it depends on the show. Seeing a band like U2 or Pink Floyd in a baseball stadium, I can understand it. 60,000 people, it's probably a hits-laden show, the people need to hear what they expect to hear. Putting a layer of keyboards through the PA isn't going to kill anyone, especially since Bono is still going to Bono, and Gilmour is still going to Gilmour.
If I'm seeing a band like.... Rival Sons, or Slash with Myles, or any of the Portnoy/Morse collaborations, I'm not there for the same thing. I want to see those men, at that moment, playing their instruments as best they can. If they make a mistake, or flub a vocal, or miss a cue, that's part of the magic, and for a musician of that calibre, it's less about making mistakes than how you respond. I LOVE band trainwrecks, for precisely that reason. The bands I've been in universally suck, and if someone flubbed a note bad, or lost time, the only option was to stop, regroup and start over. These guys are masters; they figure out a way. Backing tracks, clicks, whatever, IMO limit those possibilities.
I'm not as big a Rush fan as some here, but I 100% tolerate whatever they feel they have to pipe in, because at the end of the day, it was still Neil being Neil, it was still Geddy playing everything except the roasting chickens in the backline, and Alex playing guitar. I'm cool with Kiss using keyboards and some vocals in the back, because of the spectacle, and that's still Gene croaking out Deuce for the 1000th time, and I don't think I've seen a Kiss show where he doesn't fuck something up (the best was at Madison Square Garden on the first reunion tour; I was right in front of him, he was standing up on a riser on the right and completely lost the track of the lyrics to Let Me Go, Rock And Roll; oddly, it was an amazing moment). I'm starting to turn on Marillion a little bit, because it seems that Mark Kelly's sole job now is triggering sounds, not playing keyboards, and it's getting a little TOO perfect.