Not everyone feels all their data needs that kind of protection though. And it doesn't really apply to some services, for example, everything I do on GitHub is already public anyway.
Something like that could even be integrated into iCloud if it becomes popular. Then your "user provided server" would probably only work with other Apple products, Apple style. But for many users that would probably be good enough.
And you wouldn't be limited to having only one of these "user provided server thingies", either.