I use carbon cages and love them -- they're not a waste in my opinion. Yes, they're a tiny bit lighter, but the real reason I like them is because my bottle stays put -- it won't bounce out of the cage. They're easy to put a bottle into and to pull a bottle out. They don't stretch out like metal cages. And they don't leave icky black marks on my bottles.

ps -- I race on the bumpiest roads in the world -- central valley farm country. I also ride centuries and other endurance events.