I disagree about 11 (using "main_command sub_command:sub-sub_command" rather than "main sub sub-sub" syntax), but it's mostly a matter of taste.
Seriously, though, if you've already taken the time to write documentation, then there's no reason not to also generate a manpage. Just using pandoc to convert your, say, README.md gives good-enough results:
pandoc -s -f markdown_github -t man -o your_cli.1 README.md
(There probably are other good conversion methods.)
Why I like man:
Advantage over online docs:
It's offline and available directly in the terminal, without having to open a browser and it has a distraction-free, clean look. The only slight disadvantage is the lack of support for images, which are occasionally helpful, but in a pinch, for some use-cases, you can have ascii diagrams.
Advantages over "--help":
1. Conventionally, "--help" just provides a brief rundown/reminder of the options, so having full documentation is valuable.
2. If "--help" provides the full docs then:
a) You lose the option of having the brief rundown, which is also very valuable.
b) "man command" is slightly faster than "command --help" :p (yes, it is a slight pity that accessing the full docs is faster than accessing the brief version, if you use convention).
c) man deals with things like having nice output, with proper margins, at different terminal widths.
d) man deals with the formatting for you, providing consistency with all other applications.
FWIW I think that texinfo is (mostly) even better than man, as it considerably improves on the navigation, but it's been crippled by the FSF-Debian GFDL feud, which meant that the info pages weren't actually installed on many systems, and it's mostly a lost cause now.