I don't think the OP is still coming around, but after reading through this entire thread, I'd like to offer one more reason for staying ON-site, that I haven't seen mentioned...
The ability to split-up, with ease...
My children are 20 years apart in age, from oldest to youngest. When the timing can be worked out, we use our vacation time as a way to reconnect with each other. (One is away at college, one a college graduate, living on her own, 3 hours from home.) With children ages 6 - 26, and 2 Disney loving parents, (but one with more
get-up-and-go than the other) we like to be able to split up from time to time.
We spend most of our time together, and always eat one sit-down meal together per/day, but if we are visiting MK for a 2nd time on the same trip, the oldest daughter may say she wants to sleep in and just meet us later, or the girls might head back to the resort in the afternoon to catch a few rays by the pool. And, although I love Disney as much as my husband, I can't keep up with him ..!!!.. so he and the older kids might stay right 'til the very end of the extra evening hours, while I take the youngest back to the resort.
Also, with charging privileges and the
DDP on our room key, I don't have to worry about making sure they have money on them for something they may want/need when we split up for a couple of hours.
All this is easier when staying on-site and being able to use Disney's transportation.