To me, it's not worth it.
Off site, you can get a suite or 1 bedroom apartment, with equal or better building quality as a deluxe, just a short driving distance from Disney, for less than the price of a value.
Our most recent trip was on-site, subsidized by my wife's employer. We stayed at Yacht club. I will not lie, it was AMAZING being able to walk to a couple of Disney parks, and minimize the use of bus/car transportation. We paid very little for what amounted to an awesome lagoon view and a week long stay.
What made it bad? The room was TINY! And it only had a lackluster fridge (barely kept cold) and coffeemaker. We like to make our own breakfasts, because $10 dry scrambled eggs and mickey waffles aren't worth it to us. We like to keep a couple frozen foods for economical late night snacks after a long day at the parks. Without a freezer, oven, or microwave, we couldn't do any of that.
We like in-room laundry. The ability to do a couple loads of laundry allows us to end the trip with minimal dirty clothes, making the return from vacation less stressful.
We don't mind driving. Bus transportation is horrible in the sense that it doesn't save me any time, which is at a premium in the parks. Sure, as I admitted before, it was nice not having to drive. But is that worth $300/night more? Absolutely not.
You can argue that i'm not fighting fair since I'm only considering deluxe, and that we can easily stay in a value or get a deal on a moderate on site. But, that's the thing, the whole draw of staying on site, in my opinion, is making your trip...easier. Depending on bus transportation does not make it easier, in the sense that it guarantees more time in the parks or anything.
As a previous poster stated, it's really hard, when comparing apples to apples, to justify Disney prices, provided you're willing to spend a little bit of effort with planning, and don't consider driving a chore (I don't).