If a given transmission has regular flushes like Tech II is describing above, I see no real issue. I tend to get itchy when I think about re-using any fluid, even new fluid that was part of a flush. In my world, if fluid comes out it stays out, so otherwise seems weird. That, and you'd have to trust that whoever collects the reused fluid has a perfect way to keep it clean while collecting. I've seen the work of enough bad mechanics to NOT trust easily. I would suggest a dealership or trusted mechanic do this, not Walmart, Jiffy Lube, Grease Monkey, etc. . Sorry if I offend someone, I've seen too many issues come from those directions.

I've personally never flushed a transmission no matter the miles, I just drop the pan and filter, replace the filter, clean the pan, put it back together, and fill with new. I tend to get well over 200,000 miles out of them (and counting) unless there is other damage done or factory defects installed (like 6T75 35R waveplate among others). My record so far is 284,000 miles and that 4L60-E didn't fail outright in any way, just started to leave a bit of metal in the pan while having 284,000 miles on it.

I've heard that keeping some clutch particulates in the fluid helps with friction in the clutches, or more accurately, taking them out (by thorough flushing) makes the fluid too slippery in a transmission that has become accustomed to "less slippery". It makes sense on the surface, but I haven't seen any science that proves it. I've also heard that older fluid is thicker and doesn't leak past seals as easily. I tend to not believe this one, although I have to admit I haven't seen any science on this either. There are a lot of theories on the mysteries of automatic transmissions out there. I tend to agree with carfixer007 above when he says that these transmissions that started slipping right after a flush were probably getting close to having issues anyways. My 228,000 mile Metric 250C in a B-body between an Olds 307 and a 2.41:1 rear end did this. In this case, I call the fresh fluid the final straw, not the cause.

In the end, automatics are pretty reliable (except for some of the well-known weakling models). If we asked the manufacturers for some science like I'm looking for, they'd probably say "see that maintenance schedule in your manual? We scienced our a$$e$ off to create that schedule." . . . and while they did this, they weren't under a lot of pressure to over-extend the interval like they appear to have been with engine oil intervals over the last couple of decades. Joe/Jane average cares that he has to interrupt his/her life to get an oil change every 3,000 miles versus every 6,000 or 10,000 etc. He/she doesn't care nearly as much about a transmission service happening at 97,500 miles versus 89,000 miles .