the huge benefit in real systems for Jev like solutions i see is the cybersecurity / prompt injection mitigation. since the output will always be well structured, there is no way prompt injection might make the system do something crazy.
probably a prompt injection can still affect the output though, in unforeseeable ways.
You can use structured outputs and validate them against a schema today. I do agree making it a hard constraint instead of best practice for developers closes a whole class of bugs.