That is a pretty silly practice. You gain no advantage whatsoever from having a hotel reservations with regard to getting
free dining, if and when it is offered. In order to get free dining, there needs to be special free dining inventory available. Everyone who "adds" the free dining to an existing reservation is really being switched from regular inventory into this limited free dining inventory (but Disney doesn't bother the guest with this detail). If there wasn't any space left in the special inventory, having a room reservation wouldn't help you at all.