When I was going through my training, our instructor who was an old fire line hand, incident commander and hot shots crew member had a few things to say about water bombers/tankers. Simple of it is, as impressive as they are, they just can't put out enough to knock out a fire. Usually the fire sputters, then rolls back around and keeps going. The best effectiveness is if they can get a few of them on the fire just as it hits a break, but against a full front its not going to do much. He said a lot of the fires, they roll out the planes when a politicion is around or news crew is doing a special and thats about it. The one time everyone appreciates them though is in a potential overrun sitaution. He said getting a full load dumped on you is no fun, but in a situation like that you'll take it in a heartbeat.
Theres a lot more tankers then just 8 c-130's, but those are the big boys. They are also insanely expensive and the fire agencies are operating on shoestring budgets. Everyone loves the fire fighters when there is a fire, but the res tof the year they are just an expensive nuiscence.