Truly......if they do not want to do reservations before they go, at least if they walk up to any guest services at any resort and make a reservation for a restaurant, I would think it would be better than chancing being a walk up for that same day.
I have seen this work firsthand, I was turned away at the podium for a certain restaurant at a resort, went to guest services and got a reservation for 5 min from the time when I spoke to the castmember at guest services, went back to the podium and was seated..
I am not saying that they may not have luck with walk ups, it is just why chance if they seem to be holding tables for reservations....