Well, "only something legal" does useful things. If two persons deposit money on one and / or-account, they will, in strictest terms, remain the owner of those (possibly different) deposits until the money has been spent. Certainly useful for larger amounts. As mentioned before, one person owns a connect account and if the permissions are withdrawn, people who have put money on it can also whistle.
So a real and / or bill for more than two people can be useful for all kinds of groups. For example, people living with more than two, people with more than one partner, etc.