As Tony H says, it makes me a bit nervous to pump ATF out until I see bubbles in the line, but it's only for a second or so until I shut off, and the fluid still coating the various moving parts in the trans should keep metal surfaces apart. Especially since there is no load on the trans while you are idling in Park. I haven't tried to replace the ATF "on the fly" while pumping out, but on my S90, the fluid comes out way faster than you could get it poured in down that skinny tube, so I use the batch out/batch in method.

While installing a filter inline can't harm anything - provided it doesn't restrict flow - I don't know of anything like that being installed as original equipment, but there are probably a few out there. If most carmakers don't see the need - with several hundred million auto trannies on the road - I'm inclined to pass on it, and just go with occasional flushes.

As an experiment, I fastened a strong magnet to the plastic dipstick of the PS pump in our 940 at 140k mi. To my knowledge that fluid has never been flushed, but still looked clear and red. After 60k mi, the magnet still did not have a single visible particle on it. The PS system is of course very different from an auto trans. Anyway, FWIW....
--
Bob: son's 81-GL, dtr's '94-940, my 83-DL, 89-745(V8) and 98-S90. Also 77-MGB and some old motorcycles.