While I also dislike the existing behavior, note that you can get what you want by using an equal sign.
>>> import argparse
>>> parser = argparse.ArgumentParser(prog='a2x')
>>> parser.add_argument('--asciidoc-opts',
... action='store', dest='asciidoc_opts', default=''
... metavar='ASCIIDOC_OPTS', help='asciidoc options')
>>> parser.parse_args(['--asciidoc-opts', '-1'])
Namespace(asciidoc_opts='-1')
>>> parser.parse_args(['--asciidoc-opts=-one'])
Namespace(asciidoc_opts='-one')
I always use the equal sign, so I've never noticed this behavior before.
I wish that help would display the equal sign, but that's another issue.