In most current implementations, plugins act as an abstraction boundary. The main agent usually sees the 'Plugin' as a single tool (a 'super-tool') and delegates the specific MCP calls to a sub-agent or a controller within that plugin.
If you expose every underlying MCP tool individually to the main agent, you run into 'Context Window Bloat' where the agent gets confused by too many options. Keeping them 'hidden' behind the plugin interface is actually better for agentic stability in a production environment.