I've used IEM for a few years now in a band that does not use IEM. I made the move because I swear our Guitard is deaf - and when I say deaf, I mean deaf in the frequencies he has pounded himself with from his Guitar amp for the last 30 years. This means he thinks everyone is obscenely loud, and he is not.
I still use a full rig for a few reasons:
1. A back up incase of IEM failure, or if I turn up to a gig to find for some reason the system the PA company has supplied won't allow me to patch in. I don't get a dedicated aux send, so I take a line off of the FOH and also use an ambient mic (rode M3) over the drums, along with a split of my vocals and Bass.
2. We are a show band - visuals are important.
3. The rest of the band needs to hear me on stage
4. I spent a lot of time and money on Bass rigs over the years, I'm not leaving them home. I will often pull my IEM out mid set for a few songs to hear my rig. (usually in a song where I'm not singing).
5. Feel
6. Reinforcement/spread in the room. If I am at a bar gig, and I want to hear what the bass player is doing, I go stand in front of his rig. I assume people may want to do the same with me.