Some of the benefits of staying on-site can't really be measured in monetry or materialistic terms. Things like the atmosphere, the CMs, the service, security and general Disney resort experience can only be perceived as benefits if you appreciate what Walt Disney World Magic is all about - I guess some people don't
We always get a great 'home from home' feeling while staying on-site that we never get staying off-site.
It's a misconception that while staying in a Disney resort you get 'Mickey Mouse' shoved in your face 24/7 - this is not true. In the rooms and public spaces in most resorts there is very little 'Disney sugar overdose'. There is great decor to match the individual resort theme, great service, very friendly and helpful cast members, great pools, nice clean food courts and at the moderates and deluxes some very nice places to eat.
Other benefits:
- Free transportation to all the parks, water parks and Downtown Disney either by bus, boat or monorail (depending on which resort you are in).
- You get to take advantage of Extra Magic Hours (morning and at night).
- Free parking have already been mentioned.
- Being close to the parks also mean that you can leave the parks at mid-day for a siesta/swim at your resort and then return to the parks in the evening without having to negotiate the roads outside of WDW.
- You can also charge purchases to your room using your room key at nearly every shop and food outlet in the parks (apart from some of the smaller snack carts) - so you don't need to cary credit cards or loads of cash.
- You can arrange for purchases in the parks or DTD shops to be sent direct to your resort so you don't need to cary bags around with you.
- Some resorts are within walking distance of MK, Epcot and MGM.
- Magic Express transfers.
- Dining Plans
- Refillable Mugs to use in the resort food courts - for around $12 you get free drinks for the length of your stay.
I guess the downside is the extra cost - but you can always get discounts like AA or AP. Or, do a split stay - some time on I-Drive then some time on-site to get the best of both