you don't think you need it but when you get there, you decide you do need it. Is it hard to get a cart on the fly? Is it a lot more costly to rent it on the fly? Thanks!!
By motorized cart do you mean an ecv (aka scooter) or a golf cart like for the campgrounds? Depending on when you go you should be able to rent an ecv at the parks. They do run out on some busy days. The cost is the same and you can't reserve one. If you want you can call an offsite vendor (check the FAQ sticky on the disabilities board for recommendations). They can meet you or drop an ecv off at you hotel with they are an authorized Disney vendor and you are staying on site. The offsite vendor is cheaper than the onsite rental but usually has a minimum amt of days to rent