We do both, off site, then "splurge" once a year and stay onsite. Most people on these boards look at the cost only to justify where to stay. Of coarse staying off site can save you money. But staying onsite gets you, FOTL, You'll "feel" like you're a part of the action, great pools that have a lot to do, and you can walk back and forth to the parks.
For example, how many of you go swimming when you visit an off site hotel? We don't. Yet when we visit an onsite hotel at USF or Disney, that's the big thing after visiting the parks. If we stay off site, we just come back to the room, watch TV, and go to bed. It's not going to kill our vacation, but having it onsite is nice.
It's a "feeling" thing, and judging it by the dollar value doesn't work. Just like people who don't want to rent a car because of the money. In that case, it's a "convience" thing. The rental car lets you ping pong between Disney and IOA, visit grocery stores, and you can come and go as you please. It's worth the extra money, but many still use the shuttles because they want to save money.