I think zangocaster is mostly full of shit, but I agree with him here. I guess I am just used to being able to flinch from the floor monitors. I have used the IEM a few times on sub gigs and was nearly decapitated from the occasional feedback squeal. I tried to do the one ear thing, but noticed on the way home that the ear with the IEM was bleeding much more profusely than the one that I listened to my guitar with.
If you have your shit together you can probably get superior results with the IEM, but most people I know don't have their shit together (me included) and the risk is really great for funnelling an unnatural blast of noise into your ear.