I'd take a non-removable battery any day of the week, over one that I can pop out any time. I've never opened the battery door on my Nexus, except to turn the thing off from a gnarly overheating freeze. Every time I dropped my HTC Magic, the battery cover would come off, the phone would fly in one direction, and the battery would fly in the other. What does that mean? The phone turns off!

The iPhone 3G was the most durable phone that I ever owned. I could drop it during a phone call (happened once) and picked the thing up a second later to continue talking. You lose a great deal of 1) internal space and 2) structural integrity when you make the battery removable. Having a non-removable battery means that you can skip implementing all the rails and pieces to hold the door on, the frame to hold the battery, etc... gives you a bigger battery overall and more room for other goodies, and/or a smaller phone overall.