If I'm listening to a band, and everything's good except maybe the guitarist is a little weak, or the drummer is steady but mostly a bag of clichés, or the bassist can't do anything but thump roots and fives, I can usually deal with it. But if I don't like the vocals, I have a lot more trouble just accepting them because everything else is fine. I do consider the vocals part of the whole, but there's no question that for a lot of people, they're on a different standing somehow, often the make-or-break piece of the puzzle.
I love instrumental music, partly or perhaps mostly because of this. If the guys can play, let 'em wail, I want to hear it all. There are so many bands that sound awesome to me... until the vocals come in.