+1 to what Wayne said. Getting the same "mix" from the monitor feed to match the "FOH" is not as easy as it sounds. While it is totally possible with solid modern gear, it is not "normal" and does require a great deal of extra work for the FOH crew to achieve.....unless they are already set up to run that way. If you are one of three or four acts on the bill that night, you can forget about it.
My earlier post was only meant to show that it can can be accomplished with even simple systems.
Even with a system that has multiple monitor sends, I don't see how it would be any more difficult or time consuming to make each monitor approximate the FOH mix than it would be to give a unique mix to each individual performer.