Yeah these are all good points. And no ... a vocal is never 100% on pitch unless it's been auto-tuned 100%, and we all know what that sounds like. It's definitely not a half step off very often, and even a quarter step is pretty obvious depending on the song. But notes will often be 10 or 15 cents sharp or flat and will seem perfectly acceptable to most people. In case you don't know, there are 100 cents in one half step, so a quarter step would be 50 cents. That's pretty noticeable to most people if you sustain the note for any length of time, and it's usually pretty easy to spot even in phrases that move a lot.
A half step off is almost always blatantly obvious, but that doesn't mean there haven't been many "pro" vocals on hit records with that kind of issue. "I Want You Back" by the Jackson 5 is a classic example. Listen to Michael singing "Ooh, ooh, ba-
byyyyy" at around 1:13---it's straight up a half step off pitch when first hits the final note. Not to mention that the bass line is pretty rushed when he plays his first moving line right after the first hit of the song.
https://www.youtube.com/watch?v=-T5xWkO56GI
So, yeah, like you said it all basically comes down to what seems acceptable to people. And that's what I go by: if it bothers me a lot, I'll redo it. There's a gray area, obviously, when you include the feel of the track as well. I've left many a sour note in an otherwise note-perfect guitar solo over the years because I liked the vibe of the solo and haven't wanted to touch it. The attached song, in progress (still needs vocals and acoustic guitar), is a perfect example. The solo is in A, and it moves from an A chord to an F chord. I pretty much just decided to play A blues over most of it and not really worry about "making" the A major change. But in the last speedy lick over the F chord, you can hear me play a C# note instead of a C. I knew it as soon as it happened, but that was a first take solo, and it just had the exact vibe I wanted for the song, so I wasn't going to touch it.
Regarding Pavement, yeah it's all subjective. I know
I certainly would have wanted him to redo some of those spots if I were producing the album, but that's just me. Obviously, it didn't bother a whole lot of other people.