I can think of gigs where I wish I had used IEMs. Prime example: restaurants/receptions where you're hired to do background mood music, and the sound level of the drinker/talker/muncher/backslappers grows so loud that it dwarf's the band. In those situations we're not supposed to be the "main attractions" with volume, and turning up would trigger a pointless cycle of volume wars -- but it sure is hard to play when you can barely hear the other instruments - much less your own. You're basically playing from memory and hoping it doesn't sound like crap to someone who is paying attention. In cases like these, it's heaven to play in the isolation chamber of IEMs.