This is one of the few places I think it's obvious why MCP provides value - an OpenAPI document is static and does no lifting for the LLM, forcing the LLM to handle all of the call construction and correctness on its own. MCP servers reduce LLM load by providing abstractions over concepts, with basically the same benefits we get by not having to write assembly by hand.

In a literal sense it's easier, safer, faster, etc for an LLM to remember "use server Foo to do X" than "I read a document that talks about calling api z with token q to get data b, and I can combine three or four api calls using this http library to...."