That's all fine and dandy but Nightwish ain't got people pretending to play cellos on stage.
As far as I understand, JP pretended to sing while studio recording was being played, hence a playback, a thing that doesn't belong anywhere in concert, let alone a metal one...
I get your point, but I have 2 counters:
1. I think we disagree on the importance of the background vocals.
2. Do we have any actual proof Petrucci just pretends to sing while only tape plays?
As far as the guitar playback go... I don't know. On one hand, it sounds uncool, but on the other, if you have only one guitarist, there's no other way to play some of the cool harmonies created in studio. Once you start multi-tracking and overdubbing, there's pretty much no way to make it sound good live unless you:
1. Lose your credibility
2. Hire an entire orchestra/other members, who are not the band, honestly
In fact, when a song gets me with its beautiful use of guitar harmonies, I'm usually very afraid it won't be as good live if there's a sole guitarist (e. g. - Led Zeppelin's Achilles, DT's very own This Dying Soul)
Also, I might be remembering this one wrong, but Alex admitted to shamelessly lip-syncing live during Rush concerts while background vocals play.