I think it just depends on what is available in your area. I can get alpaca for the same cost as most wools or cottons here. Sure, I could spend mega-bucks on it but I wouldn't, at least not for a blanket.

I don't recommend bamboo. Sure it's the cool new thing to knit with but it felts really easily and not in any way that would be considered nice. (Think hard and scratchy and poky and prickly.)