I would venture to propose that BG Vocals are the single hardest thing for the normal band to do. If you don't know what to do, how to phrase, how to sing WITH the lead (both in timing and type of voice), and most especially, to sing in tune, it can be a painful thing to deal with. I will often just do them myself, to avoid the hassle.
It's usually a little better with a solo artist.
This is one of the things (just one) which made The Beatles so unique. They were awesome background vocalists (and not too bad on leads, either).
In the case of more than one BG singer at the same time, I find as much as possible it pays to have them on one mic, and to get the balance live.
For other "background" instruments, such as percussion, marimbas, little overdubs like that, a good room with a good omni capsule can do wonders.