Some people do it regardless if it is a Disney Wedding or local wedding. We got married many years ago,(not a destination wedding) and had guests send the respond cards back with additional uninvited guests, also not saying WHO the guest is. In one case the +1 happened to be someone I most definitely did NOT want at my wedding. In most cases my parents called and explained that the hall only holds X amount, so unfortunately we can not extend and invitation to (insert name) or (if you don't know who the + guest(s) are) can not extend and invitation to any additional guests other than who the invitation stated. Most people were really understanding about it. And the ones that weren't came to realize when they themselves were planning a wedding.
Good luck to all you brides with all these kind of issues. Hopefully everyone will be understanding.