I have a Defender case. My phone does not always lock into the case. It has come loose and dropped to the ground several times. When I'm wearing it, I must continually check to make sure the phone is secure.
I had the same problem, plus it bulks up your phone like crazy, the Commuter has a little bulk but not as much as the Defender, my friend had the Commuter when I first bought the Defender, I liked his and switched over, its a snug fit and comes in more colors than the Defender as well. Its actually the better price also now that I think about it.