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Actually, this is a rather common concept. Broadly used tools like for example Git use this kind of subcommand handling.
This command shows all remotes:
 git remote (i.e. is like git remote list)
Showing/removing remotes is done using subsubcommands:
 git remote show [...]
 git remote rm [...]
That, in combination with the explicit design goal that "[argparse] isn't dogmatic about what your command line interface should look like" should be enough reason to be wanting this, in my humble opinion.