@liw I don't think subcommand or not is the right dimension to make that decision. One man page (like most documentation items) should be split into sub-pages if it is too long.
So, for instance, a command with relatively few (or simple) subcommands can very well document all of them in a single manpage.
Whereas a complex command without subcommands might deserve being documented across multiple manpages.
Sorry for being contrarian!