Do oil rings really ever wear out? When I hear people say, you've blown your oil rings, what exactly does this mean? Lets assume compression is good.

I can understand how oil rings could get stuck in the lands in some cars, if people let their oil go too long leaving sludge, etc to get the rings stuck, but I dont understand how they could become worn.

If they're worn, what is this caused by. Do you have to really beat on the engine?