It would also be worth considering what this would mean if the mobile skin ever became a desktop skin. What does usage of the non-JavaScript watch feature on desktop look like? Also what about super slow connections - are we going to prevent watching there until they have loaded all the JavaScript - it's always good to have fallbacks.
In terms of article_action%3Dwatch VS action=watch - you actually want both. In case one it only catches the case when an anon clicks the watch star and then logs in/creates an account and then watches the article. Note this applies to both non-JavaScript and JavaScript users..
action=watch applies to people who are already logged in with slow connections / JS disabled; action=unwatch applies to people who are logged in and unwatch an article with slow connections / JS disabled.
I'm still not seeing the real benefit here - if it's just about screen real estate, why don't we just fix the design for this case?