No matter how loud the stage you're on is, if you take away the monitors AND put earplugs in your ears, you will have less SPL at your ears. Now, if you put what you need in your ears, and not more than that, you can make a mix that is both more effective at helping you know what and when to play, and much quieter.
When I play, even though we don't have any amps on stage, there is inevitably drum wash and some bleed from the PA on stage, but it's not very loud. Still, with my in ears in and everything going, it's quieter than that in my ears - if I take out my in ears, it gets louder.
Anyway, the point is, if you pay attention to what you're doing, you can use in ears to help save your hearing. Of course, you can also be reckless and make your own personal ear torture scenario. It's up to you.
Hints for making up your mix: Your in ear mix doesn't have (and probably shouldn't) include everything. We have 5 vocalists onstage most weekends. if someone isn't leading one of our songs, I don't have their mic in my ears at all. We often have 2 acoustics. The one the worship leader is playing is in my ears, the other one, usually not. You need timing cues and pitch cues - if someones part isn't providing either of those, you don't need or want it in your ears.