Yeah, but your typical motorglider has a glide ratio of, what, 1:25? And they presumably have had training to make use of any thermals they encounter en-route to gain some altitude without restarting the engine.
Your average spamcan will have a glide angle of 1:10 if you're lucky, and the average PPL will not know anything about thermals other than that they're bumpy, which tends to mess up the interior after a while.
Furthermore, you've got to be very careful in managing the engine temperatures/shock cooling when you do that. For an air-cooled engine a full power/engine-off glide cycle is not a great idea. In motorgliders, the Rotax 912(s) is very common, which is water-cooled and doesn't suffer from shock cooling all that much.