Good question.
I'm not sure but it could be a left over feature from an earlier version.
The RSS "Comment feed for this post" may have replaced it.

esanchez wrote:

Is this what "allow users to subscribe to comments" means?

That means "allow registered users to subscribe to comments"

I do have a plugin that allows visitors to subscribe by email, but I've no idea if it'll run on 2.x

¥

Could you share the file? I'd like to give it a try.

Edgar

sure, I'll link it when I find where I stashed it :p

¥

*edit*
Try [url=http://www.innervisions.org.uk/media/am_subscriptions_plugin_0_0_0_0_0_3.zip]here[/url] last known to work on 1.10.x