There are very good reasons why inflatable jackets are used over inherently bouyant devices such as seat cushions in aircraft.
Islander Jock, there's another important safety consideration to add to your list:
(5) Inherently buoyant devices (and pre-maturely deployed inflatable jackets) are nigh impossible to push under water to reach a submerged exit (sucks to be the last one out).
I remember the incredibly strict regulations imposed on us (naval aviation) to ensure that only non-water activated jackets were carried aboard aircraft (the deckhands all wore jackets which would inflate if they fell overboard and we had some identical-looking jackets for pax).