It seems to me that MCP alone could already allow the main use case claimed by A2A, which is an agent assigning a task to another agent: if you put an agent behind a MCP server, an agent can query it as if it was another tool, and voila, you don't need A2A. But maybe I miss other use cases.
It's not so much about what you _can do_ but about the messaging and posturing, which is what drives the adoption of standards as a social phenomenon. My team's been working on implementing MCP-agents and agents-as-tools and we consistently saw confusion from everyone we were selling this into (who were already bought in to hosting an MCP server for their API or SDK) for their agents because "that's not what it's for".
