Rapid Predictive Miner, if I understand it correctly, is meant to simplify the model building process by using pre-defined modeling process flow diagrams. If you have relatively standard problems that RPM process flow can solve well, then it lets you very quickly run some models and see some results.
If you need to tweak the process flow or do something fancier, then you need to drop into Enterprise Miner where you drive the entire modeling process, including putting together the process flow the way you want.
I think of it as auto-pilot/cruise control versus taking the wheel yourself. There are times when you'd want one or the other, and the flexibility is there. RPM isn't necessarily leaving anything out per se.
