I am quite confused on this topic.
I just had my tranny fluid changed for the first time (57k miles). Last time I was in for an OC, my mechanic mentioned that the tran fluid was getting dark. He said it didn't look that bad, but might be good for a change. So, all I had them do was drain and refill.
When I had the 30k checked at the dealer, they said the fluid looked good and didn't need anything further. So, I have been riding on it since.
I have read from numerous sources that tran flushes are not good, as they flush the opposite way against any one way valves and such. They also tend to loosen up deposits, but more often than not they remain in the engine due to getting caught on said valves.
In short, what I have read is that car manufacturers, mostly across the board, do not recommend nor 'schedule' transmission flushes due to the above points. They only go so far as to recommend a drain and refill over time, which keeps the fluid relatively clean (if done in a consecutive manner, over the course of a couple OCs, all fluid will eventually be 'new').
Can anyone set me straight?