I don't have a dog in this fight, but sometimes, companies are left with no other solutions. We had some websites hosted with Wordpress VIP. Because of their full page caching and just general restrictions, creating a login and user registration system like we wanted was not possible. We had to use a 3rd party hosted solutions that loaded with javascript(we used Gigya). Before that, I thought exactly as you do.
There is a reason Gigya just raised 25mm more dollars. Granted, they do more than just user login as a service.
Gigya looks to be a lot closer to a DMP in an advertising world than a SaaS login service. They help getting visitors to identify themselves, that's a completely different positioning and feature set.
Knowing my audience and being able to monetize that is not the same as managing billing.
That said, it does sound kind of interesting, but from my own experience, I'd probably choose to build payments and billing myself.
There is a reason Gigya just raised 25mm more dollars. Granted, they do more than just user login as a service.