it's like anything else--you have to consider your return on investment. does your bassist/keyboardist/singer/etc completely suck? if so, then you've already conceded this point and have decided to play in a band with substandard musicians.
on the other hand, if these people are talented, creative people who are slipping here and there, it's simply a matter of rehearsal. and if their parts aren't rehearsed enough, those parts shouldn't be getting recorded yet. either they need to practice more, or the parts should be changed to something they can handle.
i played in a jam band in which we had 2 hand percussionists, and they were both hopeless. it saved me a lot of frustration when i came to the understanding that my friends and i were just doing this for fun, and nothing serious would ever come of it. if i had wanted to pursue something more professional, they'd have been kicked out in a flash.
so it just depends on what you're trying to do. you can't get blood from a stone, so consider the source. and if they're just making small mistakes here and there, you can always punch in and out to eliminate a lot of blunders...
hope that helps?