Resting rocks on the walls of a tank could create different pressure points on the glass. What if your rock structure isn't as stable as you hoped and one of the rocks came tumbling down?
Another thing to keep in mind is that with such an aquascape is that if you don't have a source of flow behind the rocks (maybe thru use of a spraybar, or hidden powerhead behind the rockwork), it could accumulate a lot of detritus behind there and become a nitrate factory.
Right now, I have my rockwork as freestanding islands of varying heights. Water flows all around the structure.