If you could twist out of them and thrash your legs your chances of floating in the moving snow are greater. This assumes you don't have powder straps attached. The Rottefella TRP - 100 Releasable Plate is also just a plate that needs a standard 75mm. to become complete. If you fall in couloir and a ski comes off with brakes you'll slide to the bottom and have to hike up to retrieve it. Or, if you fall in deep powder without powder straps you may spend quite a while finding a released ski.
Releasable bindings weigh more.
There is a current trend of hard skiing, bump bashing, air catching telemarkers opting for heavier, but non-releasable bindings. It seems that this aggressive skiing style could really use the safety of releasable bindings but many do not choose that option in their bindings. One reason is that the releasable bindings have tended to be not as tough. Perhaps that will change in the future. Another choice is to have the classic three pin binding, a cable without pins, or both.