Basically because of the difference between the memory loss within rechargeable AA batteries... The fact that when you first buy the batteries, you may (for instance) get 400 pops per charge then shortly after it drops to 300 then down to 150... you get my point... The canon batteries, while a tad more than $10-15, are 1 battery vs 4, you can pick up the BP-511's on ebay for a song and a dance or you can get the bigger 5d,7d batteries for a bit more, have more charge in them, and at least with the 5d's/7D's you get a good computer chip that tells you how that battery is doing and if you need a new one... On the metz 70mz series they provide them with a rechargeable battery and that thing is a beast. Those lasted me 2-3 times the shooting time than many colleagues who had the regular flashes... unfortunately I had to part ways with that flash unit but I'd like to see that kinda battery implemented in these flashes... Even rebels have rechargeable batteries and the 580's are getting into the lower rebel price range so i'd like to see that upgrade. Heck i'm sure if I was at Canon's R&D and proposed that, I'd get a raise. =) It's a win win for canon shooters with several spare canon batteries and canon inc.