Are you going to have an OpenAPI client for your local git CLI? Now every tool you might use needs an OpenAPI compliant HTTP server? Sure, if the problem is “how to get AI to talk to web servers,” I could understand this angle. But the problem space is bigger than that, and the goals are different. So having a more effective “protocol” makes sense
Honestly, I could see a pretty easy use case for some OpenAPI endpoint definitions to be pass through to a local tool. As such, adding the ability to translate a URL to a CMD could be useful.
Outside of that, most of the effort has a very 1:1 use. Response code would be exit code, of course. But describing the output would then be the same. The vast majority of an OpenAPI doc is for documentation purposes.
And, I should add, I'm honestly not a huge fan of OpenAPI. I'm just guessing I will also not be a fan of MCP.