I never found the RSS icon too useful. I think Firefox should keep the ability to parse RSS but most feed readers have an auto-discovery feature and that's usually enough.
In sum, the icon is another example of a broken UI for RSS, which is otherwise a great technology.
Your point that Firefox led site owners away from linking to the feed is an important one which I hadn't considered.
I also agree the icon's useful for indicating an existing feed. It's easy not to think about this use, because most of us probably just notice it when we need it but rarely click on it.
Other than the icon being an indicator, I don't find the way Firefox handles feeds too useful (that mix between bookmarks and news feeds).
You can easily tell firefox to use another program (or even service) to subscribe to the feeds. I've got mine going to google reader with them. The icon has been removed from firefox 4 (I'm running the betas). I don't know if they'll put it back in by default from any amount of pleading, BUT surprise surprise, there's an extension that reimplements previous functionality. https://addons.mozilla.org/en-US/firefox/addon/rss-icon/
I did. I never actually clicked on it, but I find telling me that a site has feeds valuable, because if I'm interested in the site's content I can just paste its URL in Reader and know it will work.
In sum, the icon is another example of a broken UI for RSS, which is otherwise a great technology.