Oooh, this is a fun one!
Disclaimer: We purchase APs, but we get the DVC discount (which we are told regularly that 'could go away at any time'.)
First, to the folks who say "Well, Disney would rather have a AP visitor whose admission cost is down to $15 because of all the $$$ they spend in the park". Sorry, no. If this were true, Disney would open the gates to all theme parks free of charge and make an incredible amount of money on food, drink & souvies.
Second, some of you folks act like if you don't make a ressie because the AP discount wasn't attractive then that resort room will go empty. Sorry, wrong again. Yes, Disney would rather a AP-rater than an empty room; however, Disney's occupancy is heading up again and Disney would like a rack-rate package-payer much better than a room-only AP-rater.
Third, as srfrgrl07 (hope I got that right) points out, some folks here are assuming that every AP holder is like a DIS AP holder. Wrong again. Many (most?) AP holders are those who are looking to maximize their vacation dollar value, and off-site is by far the better value for those folks (not me - I own at WDW, so I'm hooked on pixie dust). IMHO, this scenario is much more likely: AP holders see a great rate on Southwest, make a $59/night ressie at Generic Suites Main Gate (where the get a free breakfast) and eat as much as possible off-site. And Papa Walt don't make that much from people like that.
So, while DIS-type AP holders - those folks who scan the DIS for discounts but will stay, play, eat and spend on-site no matter what - might well indeed generate a lot of $$$ for Disney, I maintain they they are in the minority, and that most AP holders simply generate more wear and tear on the parks. I think that tlbwriter makes the best point: Disney is wise in the ways of business, so if getting people to buy and use APs really made more $$$, you'd see AP promo literature every time you turned around, got on a bus, turned on the TV and opened your mailbox (both snail and digital). But they don't - end of story.
IMHO - YMMV