I would say there should always be a contract between the sender and receiver, whether that's using static types or otherwise. Not having a contract is a nightmare.
For example, say a satellite sends a number to the throttle control in feet/second, but the throttle control thinks its in m/s. To each of those systems, they're just passing a number and don't know any better.
Every JSON API call currently works without a contract. In theory it should have one, but in reality it doesn't unless the server (hopefully) validates. Either can change at any time without informing each other.
WSDL based APIs on the other hand have clearly defined contracts at both ends but there's more overhead involved.
