They aren't always available, but when they are, AP discounts at the resorts can be good enough to make buying an AP worthwhile even for just 1 trip. Over the past couple years, it seems like the average AP savings on rooms has been around $20-$25 per night for values, $40-$45 for moderates, and $80-$100 for deluxes.
And depending on how you like to dine while at WDW, being able to buy the DDE card can save you a lot of money. We took a 2 week trip last March, staying offsite for 4 nights, at ASMo for 6 nights (2 rooms), then at POR for 4 nights. We ate quite a few table service meals, and between the DDE and the room discounts, we probably saved close to $1000 from buying 1 AP.
But everyone's situation is different, so you'd have to do the math. Just remember that AP room discounts aren't a guarantee. I doubt Disney will stop offering them, because regardless of what anyone says, it is one of the reasons a lot of people have AP's. But you never know.
You also get free parking at the parks with an AP, so if you are someone who stays offsite, that saves you some money.