By now, MediaWiki is severely crippled without the API, with most of interesting extensions relying on the API, it becomes more and more problematic to disable it. Now it's probably fair to say that wikis that do that are just shooting their own feet off for no good reason. Therefore, I propose to officially declare that MW does not support working without the API and remove this very setting.
I'm gonna start a wikitech-l discussion, opening this bug for tracking.