I'll concur that once monitors are set, there is very little that needs to be done as far as adjustments go. FOH mix can be tweaked throughout the night, but during songs, you want to do very little tweaking of the mix on stage. The artists usually will work with the mix that's created and play along with that. If you make changes mid song, that's going to mess with their performance.
That being said, I still put my more experienced personnel on the monitor mix. When things go wrong, there's more to have to process in your brain as the monitor technician. The FOH guy has one mix to deal with. The monitor guy could have a dozen.
