Dont carbons lose spine after a year or so of regular use? Aluminums dont, right? Well I dont have enough money to drop $100 on arrows every year, plus have to throw half of them away because some of them arent the right spine or whatever.
Aluminum will also eventually degrade over time and continuous shooting. If you flex something like an arrow enough times it will eventually begin to degrade over time.
The price thing is a non issue any more. There are way too many high quality carbon arrows that cost about the same or less than an aluminum of the same straightness. A gold tip XT runs about $80 per dozen and a XX75 Real Tree runs about $70 per dozen fletched with vanes.
Manufacturers are constantly improving the techniques of making carbon shafts. Spine is just as consistant on many of the newer carbon shafts as it is on aluminum shafts.
Many carbon arrows are weight matched to 1 to 3 grains. Aluminum shafts are weight matched to 2 - 5 grains.