I have no problem with using digital manipulation (or even good old-fashioned punching in) to fix an accident in an otherwise worthy performance for which there is otherwise no real reason to re-record. Whether the accident is accidentally bumping a mic, just plain missing one important note or chord, or a temporary goof behind the glass doesn't matter; if it's otherwise a "keeper", I see nothing wrong with fixing one or two small errors or variances.
Where I do start to have a moral or ethical (I'm not sure what to call it?) problem is when the editing is meant to *create* a performance that was never really there. If that's going to happen, then the engineer is actually creating more of the performance than the musician did, and the engineer should get the musician credit for that track, IMHO. Of course the loophole in that idea is when the "musician" and the "engineer" are one and the same self-recordist

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It just seems to me (totally IMHO YMMV MIRV and all that) that there is a line that should be drawn between the guy on the musical instrument who is capable of laying down a decent track, but just needs to make a small fix as described above, and someone who could do a hundred takes and not get it right without having to "tighten" the whole thing up in post, in which case I have to wonder why they are recording or being recorded to begin with.
As far as the metronome/click track thing, unless the click track is actually going to be a rhythm track
in the song (don't laugh, it's been done on rare occasions), it can provide a false sense of security to have everyone play to the click separately to that rather than to each other. Because everyone is human and no one is perfect, each track will "dance around" the click in an acceptable, maybe even pleasant way. But they are all doing their own individual dance, and when you put them all together on the same dance floor, they just bump into each other and step on each other's toes. Tightening that mess up can be a real pain.
But when they play to each other, by starting with the rhythm tracks (drums or maybe drums and bass together), then playing the gits to that, and then the keyboards to that partial mix, and then vocals (or some strategy similar to that at least), they all tend to dance to the same groove. Yeah there will still be small timing issues here or there because we are not computers but flesh and blood, but - assuming the guys actually know how to play halfway decently - the timing differences will be more coherent, more "musical", and probably more pleasant, possibly to the point where digital "tightening up" may not even be necessary or wanted.
G.