If we're talking about an average Joe park guest here, how would they know that they may be "wasting" a reservation and that it will not be guaranteed to be open. If they see it listed to be selected, the only thing they are thinking is that they can ride it, that's it.
Also, it's never a waste if you have a FP+ selection. If a ride goes down during your FP window, they put in a FP+ in the system that you can use to ride any ride within the park with (actually, it automatically happens to anyone with a FP scheduled once the ride goes in the system as offline). At least that's been my experience.