I just wanted to respond to the original question. Why off-site? For us, money is not a factor; we just want the best possible vacation. And the off-site hotels are SOOO much nicer. Not even a comparison.
After staying at the Grand Floridian, we decided to experiment with the off-site experience and stay at the Ritz Carlton. We will probably never go back to a Disney hotel. The only advantage of the Grand Floridian was the monorail access to MK. That's it.
We pay more for an offsite hotel because the Disney hotels are just not that nice (imho). It is shocking to me that people will pay $700 night for what is essentially a 3.5 star (maybe 4) hotel. Just for monorail access? And there is no transportation advantage for any other park, other than MK.
Everything at the Ritz Carlton was better. The rooms, the food, the spa, the service, the pool (it shares a pool with the Marriott, which has an amazing lazy river). In short, if you are looking for luxury or pampering, or relaxation in addition to the parks experience -- off-site is the only way to go.
But what about convenience/proximity to the parks? If you rent a car (which we do), it is just as easy and fast to get to every park. Although the monorail hotels have an advantage on getting to the Magic Kingdom -- the advantage is quite slight. When we go to the Magic Kingdom, we get breakfast reservations at the Contemporary, so we use its parking lot for the day. And voila! Now we have monorail access to our car (although you can also easily walk to the car), bypassing the TTC.
Now that FP+ advance reservations are available for Disney resort guests, that may offer an additional advantage for Disney hotels. But I'll see how our trip next month goes using the kiosks in the park. I doubt FP+ will tempt me to stay at a far inferior hotel, in my opinion.
As a side note, the Grand Floridian is possibly the noisiest hotel I've ever stayed in. Like sleep-defying noise.