There's a lot of benefit to being able to state what keys may be specified in a certain location, though. Look at DSLs like Cloudformation, for instance. Having schema validation could make static analysis of this kind of code much easier to handle. E.g.: Fn::Sub may be used inside of Fn::Join, but the reverse is not true, regardless of the types "returned" by each. It's certainly possible to validate via the api, but being able to do it in my editor will make finding errors much faster.

To your other point, however, dynamic code generation is becoming much more common. AWS generates a huge amount of its code from JSON definitions across multiple languages to keep its SDKs up to date. I could see schema validation being valuable in this domain as well.