You have to register your app with facebook, thus this feature could be removed from all new apps while still preserving existing functionality for apps registered before the feature gets removed (if it ever does get removed).
Fair point, and hopefully this qualifies under Facebook's pledge to maintain backwards compatibility for 2 years.
But of course there's also the issue that this becomes something that developers themselves can't easily remove once they add it to their app. Or worse, if Facebook decides to block a developer's app from platform. At least with regular Facebook login, most developers requested Email so users could always request that a password be sent to them as an alternate login mechanism.
Developers should be mindful that Facebook explicitly prohibits using Platform to build "competing social networks" (An unfortunately broad category): http://techcrunch.com/2013/01/24/my-precious-social-graph/ And they have a history of enforcing this with the bans of Yandex, Wonder, Vine, and Voxer.
