It's also good for learning how to fall.
It's easy to see how you can break a wrist or hyper-extend an elbow trying to catch yourself. You just have to make fists and roll with it like you would on the hill I guess. I tweaked my elbow pretty good this morning to re-enforce that lesson...
Yeah, obviously you can't slide but static exercises are good. It gives you a feel for jumping with the board on, teaches you to land slightly on-edge instead of flat so you don't just slide off (I did some pratfalls the first couple of times) and teaches balance. I've done 180s on and off, hopping back and forth from nollie to ollie, stuff like that.
The workout is very much like pliometrics too. Very explosive movements. I got out of breath really quickly at first, but you get better at it.