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.

Subscribing to an issue to see how it plays out and supporting the contents of the issue are two majorly different things.