A change of types would almost always accompany a change in semantics.
Wouldn't you need to redeploy both anyway when you have a change of semantics? If you retain the old API with the old semantics, you could of course retain the old type, too.
No, you can add a field on a server that some clients do not yet use without changing the semantics for the clients that do not know about it, for example.
The loosely coupled model is more or less Duck typing for interfaces.
