I think a lot of cars have some kind of lockout for it. I have a VW, and there's a electric trunk lever in the driver side door, which has a keyhole above it to disable it. Both sides of the rear seat also fold down to allow trunk access, and both sides also have keyholes to lock them out. So I guess you could keep the valet out of your trunk, if you actually remember the valet key and go to the trouble of locking 3 different locks. Which gets to your second point, that I don't really have anything there worth caring about anyways. Maybe if they really want to steal my spare tire or something.

But then again, I don't really like valeting my car anyways.