Well, you do have to leave the extension running, but it does the same thing. As mentioned on the site, this has the benefit of offline reading capability because the rss content is saved locally. If the feed in question only contains links or a synopsis this is not much help, but for feeds which contain the full article it can be very nice. I'd also like to integrate readability.js to cache a readable version of the link for feeds which do not contain the full article.