Heres the thing with ATF flushes...most people have them done at Valvoline, or Pep Boys, Firestone, etc. These shops park your car in a bay, turn the car on, and hook it up to a machine that pulls old fluid out and pumps new in. Of course new lubrication in a system will have positive fluid lubrication. However an automatic transmission is a intricate series of small passageways and valves which direct fluid based on throttle position, engine speed, etc. To properly flush a transmission, you need to get new fluid into these passages and in the same process remove old fluid and any deposits which reside. This cannot be done with the vehicle in park...it needs to be lifted off the ground and run through all the gears while fluid is pumped in and out of the trans.
I think a lot of information regarding these flush services is very mixed based on the fact a lot of people confuse causation with correlation. Since most people only perform service on their car or truck when there is a problem, for example a transmission shifting poorly, they decide to try and flush the system. Initially this may relieve some of the poor performance of the system but when the system fails a month or so after the service, they blame the service. When, in reality the system should have received a major overhaul or part replacement.
So if your car or truck has limited miles (<100k) and the transmission has no issues a flush will not cause any damage but if the transmission is older and shows signs of poor performance a fluid change MAY relieve the symptoms but cannot replace worn clutch packs or other wear items.
May family takes all our vehicles to a shop which specializes in transmission and driveline services. Which means rather than order a new trans like the dealer will do, they will take your trans out, rebuild it, and put the same case back in with new parts. I got all this information from the service manager at the shop.
This is coming from experience working at Valvoline and a Nissan Dealership/Service Center.
Just my 2 cents.