Hmmm...people seem afraid to put even light cargo on the roof, but not afraid of having a few hundred pound air conditioner sitting there all the time. (Granted, the roof structure is sometimes built with some extra reinforcement around the air conditioner's location.)
I don't carry anything on my roof, and my roof rack such as it is consists only of a toe rail around the edge of the back third or so of my RV. It's not so much that I fear damage to the roof structure, as it is that climbing up and down is a pain and I have plenty of storage space in more convenient places on my rig for the stuff I want to carry.