Having just flown in a recent Cessna 172 today, I can see the usefulness of some means of cutting out since the releases are on the outside near the door. If a crash impact were to deform the cabin in such a way where you couldn't get your hand between the door and seat, there's little other way to get out of the belt, especially if the inertia wheel(s) are still locked.

Cars may not be a huge issue, but certain planes might be.