Like I was saying, it depends on how much backward-compatible your vendor wants to be. I just don't see how a hosted app is going to protect you from this kind of thing. The only difference is that the hosted app is presented with a "native" API as opposed to an RPC API, but it's just as vulnerable to deprecation, and will remain so as long as you are a third-party.

Except that the cost in term of infrastructure and ongoing maintenance (code review, etc) is higher, which IMHO makes it likely to be phased out sooner than a web API.