I think it is partly a mental thing. When the Rancho came out, it was very flat compared to the Ontario and was a much better disc. So "flatter is better" became a Roc way of life.
Also there is no answer to the question "Buzzz is to Wasp as Roc is to ___." When my friends that throw the Buzzz want a little more stability, they throw a Wasp in the bag and go on with their lives. When a Roc guy wants a little more stability, you do what? Try a KC or try a Ching Roc, I guess.
Anyway, the guys who throw them are very loyal to them. A heavy Sentinel MF does the trick for me, so I could never figure out what the big deal was. They do look cool, though.