Like others have said, you don't need to spend money, but Disney has a way of making you want spend money. Those Magic Bands are great at making you spend more money then normal. Beep then pin is surprisingly more easy then pulling out a wallet, finding a card, swiping the card, putting in your pin, putting away your card, and putting away your wallet. I know it seems trivial, but it makes a significant difference in spending habits.
On my trip in June we payed for all meals and other random stuff with our debit card, and we still managed to rack up a $1200 magic band bill in a week... Oh, and that was our second week there so I have something to compare it to. During our first week we stayed offsite, spent the same amount on food and other random stuff on our debit cards, and did not spend the extra $1200. You would be surprised at how fast the little charges add up. We had the money to spend so it wasn't a problem at all. I suggest putting extra money in your budget so you will be prepared for the bill.
Oh, and the dangerous phrase... We can send it to your room for you so you don't have to carry it. Disney is great at removing barriers to spending.
Staying on Disney property also talked us into buying DVC... I go to a
Disneyland weekly and I never thought of buying DVC before this trip to Disney World. (Bounce back offer was tempting too...)
Hmm... I guess staying in Disney World makes you want to spend money period... Be prepared for it!