I don't know if you can use the same email for different handles, but with gmail for example you can produce an infinity of different email addresses.
So you can't use it if the person you want to "follow" is already registered.
Now, I tried "pg" and it subscribed me. The result of this is that if "pg" wanted to subscribe, he wouldn't be allowed.
I tried to unsubscribe. Unsubscription is e-mail based and not handle based. i.e: You can't unsubscribe from just one user, you unsubscribe from all the users, since you're taking out the root (e-mail address).
Now I have to subscribe again to get my own replies.
PS: Been notified of your reply by HNNotify :)
It would make a lot of sense to allow handles to be monitored by many different people; some users are super popular and I guess many people would like to "follow" them...
-1- Being able to follow a thread you haven't commented on. -2- Being able to follow a comment that is not yours.
The reason behind number 2 is that sometimes, you see an interesting comment and you'd be interested to know what people think about it, so you'll follow it and be notified whenever people reply to it.