BlackLexRX

You actually make a good point; why run old fluid through a new filter. Conversely, full flushes all at once can sometimes lead to problems too so a series of 3 drain and fills are recommended as the safest way to replenish the fluid. Depends on how many miles you have on the fluid really. Mine was flushed at the dealer before I got it at 40k. I'm at 75k now and have done one drain and fill (3.5 quarts), was planning on pulling the pan and doing the filter around 100k as my fluid looks pristine and trans shifts good. If you are over 100k on original fluid, just to be safe, I'd say do a series of 3 drain and fills and do filter on the last one.
So.... drain/fill (3.5 quarts) drive a bit, then 2nd drain/fill, drive again a little, then drain and pull pan, change filter and fill one last time. 10.5 quarts fluid exchanged total. I think you'd get around 9 quarts if you drained all at once, which obviously would cost less and take less time. But drain and fills are the tried and true safe way to replenish fluid slowly and effectively.