I myself also love reading man pages, but speaking of compatibility, I have to say that “--help” is a more universal way of showing help pages. Of course it’s better to have both of them though.
--help is fine, but almost never a substitute for a full man page, except for the most trivial of applications (unless your --help is as complete as a man page, in which case... good on you for providing full documentation, but I'll hate you a bit every time I unthinkingly drop two hundred lines of text in my terminal.)
I strongly suspect that --help is on the contrary less universal, given that there exist entire toolsets lacking the --help convention but having manual pages. There was almost a quarter of a century's worth of Unix tools that developed before --help was invented.
