If someone doesn't want to use google, he can log in with facebook
What if they don't want to log in with either? You've already answered your own question. A separate user/pass should be allowed because some users don't trust services with access to other accounts. A service should honor that for whatever reason that user has because that's a good experience. Logging in with a third party account should be seen as added benefit or enhancement.
It should not be mandatory because it blocks users that don't have Facebook, google, twitter, or whatever other third party you think the user should have to have to be able to log in.
Additionally, I see other issues with even using third party login to begin with which some of your users might have.
- What if the third party is deprecated? How will I reconnect with my account in the context of your application
- What if I lose access to the third party account?
- What if I don't trust your application with the permissions signing in with the third party would entail?
- What if I just don't have or don't want these third party accounts?
If you only gave me the option to log in with a Friendster third party log in, I'd be pretty irked and opt not to use your service.