This thread puzzles me.
I always run at least two monitor mixes on stage when I run sound for bands.... sometimes up to 4 mixes.
I find that if each person can hear their instrument along with the vocals in the mix, then their stage volume tends to be more manageble. Nothing is worse than the bass player melting his calves and the faces of the first four rows when the bass is much more tunable and spreadable through the 18's in the main mix.
The only time I've had issues was when sax and trumpet players wanted more of their own mix but their sound was slicing the heads off of everyone else on stage.