This way, if facebook ever down (or removes their services), you have an _out_ for your users to login via the traditional username/password.
A lot of people have 6yo+ facebook accounts. I quite enjoy the fact that my verified facebook email goes to a dead account that I only monitor on a 6 month basis... it saves me from a ton of spam.
To me as both an app user and developer, single sign on feels like the Right Thing To Do. I'm sick of running through the same sign up flow for every single service I use, like a hamster wheel. Maybe the solution isn't fb, it's some kind of apple or google SSO? Why aren't AAPL and GOOG pushing their own SSO hard on the mobile ecosystems they control?
The solution is definitely not Facebook, Google, or Apple. They all plugged SSO into an existing platform and allow developers to perform actions on my behalf with the same credentials. Sadly, Microsoft's Passport.net was the closest anyone has gotten to a real solution because they treated their own properties as just another consumer. They killed that by trying to tie it directly into your Windows login which are inherently insecure.
because neither of them has a controlling amount of the required market (which is basically the entire internet!).
Mozilla has a project called persona (http://www.mozilla.org/en-US/persona/), which aims to solve this by splitting the login from verification and authentication, but preserves privacy (unlike oauth). If vendors of services can give up their desire to control the userbase, this could be the solution.
A pet peeve of mine is when I click on the login with facebook button and the site still makes me fill out my name, username, email address, and set a password. So what's the point of logging in with facebook?
Then why are they bothering to sign in with facebook at all? It is now a more involved process than simply signing up with email/password, which is what facebook logins are supposed to be for avoiding.