I've had the clear case on the 12PM and 13PM. It's always been exposed at the bottom, the case material is harder and isn't as flexible as the silicone ones, so one of the edges needs to be "open" to allow it to flex enough to pop on and off of the phone. If it was complete all the way around, then the risk of breaking/cracking would seem pretty high.
Also, for the 12 months at a time I've used the current model phone with the clear case, it's never yellowed which is a plus, but admittedly at times it can feel slippery. I'll likely not go with another clear case this year.