Carbon/aluminum mixes in the frame are pretty silly, IMO. You can engineer either one to give the characteristics you want. The combination is pretty much about marketing - carbon is the current wonder material, and it sells. Part aluminum, part carbon frames allows a customer to get Carbon, the Wonder Material, at a lower price point, and the manufacturer to put an added mark-up on a frame that isn't necessarily any better than a full aluminum frame. I like carbon just fine - I have a very nice carbon fork and carbon brake levers on my road bike, and if I had the money, I would happily ride a full carbon bike. I don't especially want to pay a premium to put carbon where it isn't needed in my aluminum frame.