I'm referring to people who explicitly subscribe to a given issue. It's true that this would not allow people watching a repository to explicitly register their

I'm referring to people who explicitly subscribe to a given issue. It's true that this would not allow people watching a repository to explicitly register their interest but I think the percentage of interested parties who are watching a repository is quite low. (GH could estimate this by checking +1 comments on issues for watchers.) I think the advantage of using subscribers is simplicity. Adding another feature that is arguably similar to the existing subscribe complicates things.