Remember my trans pressure line failure/fire last year? (June I think). I lost 7 quarts in about 30 seconds (at speed) on a bridge I couldn't stop on. I think the remaining fluid was destroyed by heat/friction. No apparent damage to the trans, though. Then I added seven quarts, and only diluted the problem.
This sucker should have been flushed then. 85k original miles on the trans. Runs like a top now.
ONLY a FLUSH will get all the fluid. You'll only get about half from dropping the pan. If you do that twice a year, you might do ok, but a flush is worth it. Cost me 80 bucks.
If I had to do it again, I'd drop the pan, change the filter, fill it up again, drive it for a month, THEN go get a flush. The flush doesn't deal with the filter, unfortuneately.