As a trivial counterpoint, streaming outputs and proper signaling support is required for being able to compose efficiently under any setup/harness. The more layers of abstractions you add, the more likely it is your MCP server didn't support the usecase that some later agent came up with and so you can't stop your stream when you find your top 3 hits. If you don't even own the server, you can't even modify the source and restart it to fix it.

There are usecases of MCPs (e.g. serving outside customers via proprietary tools), but composability has not been in the minds of the early users of MCP and it is one of the hundreds of little practical drawbacks they have when used in house.