designed and laid up for river use are not going to be "quickly destroyed" by granite and sandstone. However, I quite understand why outfitters often prefer not to supply Kevlar boats. ABS and poly are better when some renters are careless, thoughtless, etc.
A "Kevlar" canoe designed for rocky rivers won't be all Kevlar like a Wenonah, but will have E-glass or S-glass for the outer layers and Kevlar for the inner layers. I have several "Kevlar" boats, used on whitewater all over the nation. They are not easy to damage, but are easier to repair than my Royalex boats.
Light weight is one goal for serious composite boat builders. One doesn't need a 6 mile portage to appreciate light weight. Just a quarter mile uphill carrying the boat from the Chattooga to the parking lot can be reason enough. The other reward of light weight is boat responsiveness when carrying out a series of complex maneuvers in the rapids.