With an AT if you let the fluid get trashed and flush it out, you're likely to free up some deposits in the transmission and that can cause problems shortly down the road. AT fluid has great cleaning properties for the transmission but that can cause trouble with a lot of miles on the vehicle and old fluid. I purchased my 1998 Jeep Cherokee XJ in 2003 with 120k miles on it. Most people said don't change it since it had never been changed at regular intervals (mine looked horrible). I changed it anyway (yeah, I'm smarter lol) but I did several drain and fills of the AT fluid pan. It was a gradual change of the fluid over the course of a year (each oil change) and never had an issue. The AT was going strong when I sold it with 185k on the clock.
On my S2000 I change the MT oil and differential oil once a year (about 9k miles of use). The MT gets a little notchy after about a year and new fluid solves the issue. Since the car is up on stands already and it's only 1 quart, I throw some new Mobile 1 gear lube in the diff. Never had a single issue with that.
To each their own, but I disagree that the fluid should be left alone for good. Things wear out and particles become suspended in fluid/oils. Also, water can be absorbed into the fluids as well and that doesn't help. If I planned to keep my Z past the 100k mark I would probably change the AT fluid every 40k miles or so. However, since I have the extended warranty and Nissan calls for no replacement, there's no need. I factored this into the cost of the warranty as it technically reduces the cost of the warranty since I'm getting out of a couple AT fluid changes (again, because I think I'm smarter than the OEM
).