Exactly. Which is why I'm hesitant to adopt it right away. Why develop a feature that only a tiny fraction of users?
Even so developer adoption can be encouraged. By doing most of the work for the developer: a rails gem that integrates with Devise; a Symphony component; a WordPress plugin, etc.
As a web developer, I'm not spending time adding this to my site until a sufficiently high number of users are using. As it has no value beyond login itself, users will not adopt it. Because users have no adopted it, there's no reason for me to implement it. Because I have not implemented it, there's no reason of users to adopt it. Because…
…you can see the problem.
It's a neat idea, though.