I think this functionality would make sense for future usages.
For example: when SSO is integrated into Ghost (later this year?), and supposing a publication is using something like Discourse or Cove as its commenting platform, a reader may want to be a subscriber so they’re logged in for commenting purposes. However, they may not necessarily want email notifications of new posts (perhaps they use an RSS reader). A user-facing toggle would therefore come in handy.
Edit: makes sense for future-usage with Discourse (requiring SSO), and present-day usage with Cove (not requiring SSO).