NIM batteries don't like heat. The fast field chargers tend to heat them up a bit more then they like. By fast charging them they just tend to have a shorter life. Instead of three years you may only get one or two years out of a pack. That's the easy answer as to why people usually don't fast charge them all the time. I charge at home over night on some of my planes that are hung up and I can't trickle charge them. When I'm at the field and making it a full day of flying and my batteries go down I use my field charger, I really don't care if they poop out on me a little quicker then they would if I always slow charged. Fast charging works but you won't get the full life out of a pack if that's how you always charge your batteries. In my case, BIG WOOP!!!! I'm not going to quit flying because my batteries are low.