The advantage I see for MCP over CLIs or APIs, is that once you release the CLI/API and people start coding scripts against it, your interface is set it stone, you're never allowed to change it again, or you break everyone's scripts.

With an MCP, agents are very tolerant of changes, since each usage is fresh with no prior knowledge, so nothing to break. This allows you to experiment and iterate on the MCP, see how people use it and what gaps you still need to cover. Once it stabilizes, you can lock it down into and API/CLI interface.