It doesn't make sense: everybody who have a Facebook login also have a gmail, so why not not just allow people to login with that?
As for why not use OpenID? Personally, I don't want to tie my Gmail account to a lot of things -- then people have my email! I don't necessarily want to tie my Facebook to them, either, but in the same breath some forms of OpenID can end up sharing more, depending...
This is why I get cranky when people implement Disqus, too -- old comments I made that were supposed to be anonymous but which had my email for the author, etc, are now pulled in and linked to a general identity months or years later. :(
(not criticizing BTW, just curious)