The problem with stereo vocals is that it gets distracting at times.
You pretty much want the vocal track straight-up, dead center in the mix.
And what happens if you give the vocal any sort of stereo width ... is that if the singer doesn't stick his head in a vice or something, you get this very subtle, unfocused quality to it. Any time the vocalist "works the mic" in any fashion, you lose a little of that center focus as his head moves slightly to the left, to the right, or back. Basically, if the vocalist is in any way animated and likes to move around at all when he/she tracks, then you might run in to some issues.
Another issue is that you just have to be recording in a really good-sounding room, because it's going to play an even larger factor if you're stereo micing anything.