Trying to spec this, here is a proposed API:

    parser = argparse.ArgumentParser()
    sub = parser.add_subparsers(default='show')
    sub_show = sub.add_parser('show')
    sub_add = sub.add_parser('add')

If default isn't passed, the subcommand isn't optional.
If default is passed, and no explicit subcommand is given,
the default subcommand is picked.
Arguments are given to the top parser; passing arguments
to the subcommand requires naming it explicitly.

As far as motivation, I'd like to change a program that
uses --choice options (that can have a default) to use
more expressive subcommands. Some programs rely on implicit
subcommands a lot; the ip command on linux is a good
example.