Nope, it makes perfect sense to me. Open ID made no sense to non-computer enthusiasts (and to plenty of the enthusiasts as well). Those same people all have Facebook accounts and probably visit Facebook multiple times a day. They trust Facebook, so clicking on it seems painless - and it's definitely effortless.
Stackoverflow does exactly that. They have big buttons for all the major identity providers, including Google, Facebook, etc. and a text field at the bottom for manual openid login.
Personally I feel a lot better about Twitter + OAuth than FB, but that's just me.
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. :(
Why would you feel better about Twitter + OAuth than FaceBook + OAuth (which is what the sign on component of FB Connect is)? From a technology perspective, it's the same protocol. Neither company has a good security track record.
But that isn't really an issue if you want to accept open ids on your site - it is actually a good thing, because the more providers there are the easier it is to find on that has your customers.