The OBD-II port allows access to the life-safety systems of the car. It is a private unsecured network that performs no authentication.
These dongles allow arbitrary access to the car bus, limited only by their buggy software. They shouldn't even be manufactured.
I'm sure that car manufacturers would agree with you as this would then make third party products (including diagnostic tools), 'unlicensed' (by the manufacturer) mechanic work (including do it yourself) and so forth difficult to impossible resulting in even higher manufacturer prices for the same thing.
So no, I'll keep that open port open, thanks just the same.