Reading OM's article "My web without Facebook Connect", I was reminded of an blog post I recently wrote and regularly reference ("Facebook as a conversion tool: registration flows."). OM's point is that so many sites now use Facebook as a registration / login path that it has become an integral part of accessing the web: "a day without Facebook, is quickly making the web unusable."
Inverted, that also says using Facebook within in the registration and login flows is important:
1. it's becoming universal and therefor familiar 2. users are more likely to pass information through via Facebook than to a new site 3. it expedites the flows and therefore improves conversions 4. it about more than efficiency. For instance: you can add the facepile for conversions, mobile flows can occur with a phone number (rather than an email address), etc
And from the consumer perspective, I prefer it: it's easier / faster and I can change passwords / access universally (more secure).