I think the whole issue starts with when you take ownership of the vehicle. If you've owned it since [nearly] new, then you can do fluid changes once a month if you like and always have a great running trans. I think it gets trick when you buy a higher mileage vehicle, and you don't know the service history. That's when you can get into a situation like Matt described. Also, for a trans flush, I think the machine pumps the fluid in the opposite direction of the normal trans pump. It's not recommended by Chrysler because that would mean the fluid is backing out of the filters sending those particles back into the trans. Other vehicles don't use filters, but screens instead. Supposedly, trans flushes are OK with those types. Then again, I'm no transmission mechanic. This is just what I've heard/read/made sense.