I don't think flushing is a bad thing, but why isn't it recommended in the maintenance schedules of most vehicles? In many cases, I think it's one of those service practices that's offered up by garages because the cash register likes it.
On another forum, I posted that I thought it was mainly a "gimmick-sell" because most maintenance schedules don't include it. They say to change the fluid at recommended intervals, but don't mention flushing at all. One member scanned and posted his entire maintenance manual to "prove" they did recommend it...but he didn't read it real close, because it simply said "change" as well. The only time "flushing" appeared, was in reference to the cooling system. If they recommend flushing
anything, it's called out specifically.
I overhauled automatics for years, so I'm not unfamiliar with their inner workings. I just think that had the owners simply dropped the pan and changed the fluid as recommended, most of those transmissions would have never ended up on my bench. (Well that
and sticking with recommended towing guidelines and such.)
A vehicle sitting still in park with a cooling line loose will get out more of the old fluid than just dropping the pan, but if you're just sitting there in park idling there's not a whole lot going on. Servos, pistons, clutch packs, valve body ports and pathways, etc. will be sitting there essentially doing nothing.....and as such won't benefit from the flushing. Show me a flushing apparatus that allows the trans to be shifted through the ranges under a light or no load simulating and easy cruise down the road to
really flush things out...and that would be another story. That's not how it's done however, and if it were done that way the sign outside the garage wouldn't read "$99 Trans Flush Special".
For a DIY-er, I think sticking to recommended
change intervals is more than adequate. Reading the actual owners manual and heeding payload and tow ratings is a good thing too.