[sneakymichael]

Ease. If you're already logged into Google, it's essentially a one click process.

[ekianjo]

I get that. But I also don't all my services to depend on Google at all, even if it's just a login.

[sreevisakh]

The authentication service should ideally be under the control of the user. At least, the user should be able to choose one that they trust. I doubt it's an accident that current authentication systems lack that choice.