the cast wheels are not glass!
Often the problem with off road is that the riders let air out of the tires for better traction....so when you hit somthing hard the tire collapses and the rock/whatever hits the rim. If you keep the tire pressure up you have to hit something hard enough to collapses the tire...ie much less likely.
You will note in the picture above...the tire is also badly damaged...from that collapsing
If you ride in 1st gear it is unlikely you will damage your cast wheels...or your tires.
get out there and do it ....have fun
If you find you ride hard enough to damage your tires and wheel, you can always upgrade to after market wire wheel ...which are evan tougher than the BMW factory wire wheels.