And in my experience the sub agent performance is usually worse than just a single agent.

I find it useful for code reviews (spawn a subagent with minimal/no context to review X commit). Of course, this is more or less a shortcut that could be done with a seperate agent. Another use is multiple reviews at once if tokens are not an issue, with seperate "personas" or focuses. As far as implementation goes I have not seen any major usecase.