Graal is not the only app that uses UDID as a login for accounts.
If your UDID is in the hands of someone else, they can hack into your accounts on any other app that uses it to identify you. And if that app is used for shopping (with your credit card details recorded in it) or anything else that requires your personal information, then you will have given all that information away too.
Most people don't understand that UDID isn't simply Graal's login system, it is your device's and some of its apps' complete identity.