The important part of MCP (that this misses) is that it's a machine interface you can quickly iterate on. In other words, an API is too slow-moving. You can't break it; it's a promise. A UI is too annoying for an LLM to deal with. An MCP is the perfect middleground: speaks JSON, but there's inference involved, so if you change the tools, the LLM will be just fine. (Just like how you can change your UI much faster than you change your API, because there's inference at runtime, ie in the human brain.)

For example, at my startup val.town, our MCP server is way more powerful than our API, because we added a bunch of tools to it willynilly because we're not worried about having to be stuck with those tools forever. We can remove them and nobody's code will break. (Just like we could remove buttons in our UI.) But an API change needs to be thought-through.