Some folks put them up high on their ladder with a bracket made for a Tailgater ... not that crooks can't climb. Plus that makes it stationary too. I had the same questions going through my head before I bought one. Even using a lock with a security cable it can be cut through very quickly and easily. If I leave the PC for a period of time, I do unscrew the cable from it and set it inside. After way too much time worrying about it, I have finally decided that if somebody wants something bad enough they will find a way to steal it.
P.S. My dish has strong Karma attached to it ... what goes around comes around.