Shawn Blanc, elaborating on Mountain Lion’s removing support for RSS in Mail and Safari:
I still check my RSS feeds on a regular basis — as do many who read this site, I assume — and there is a huge market for good RSS clients. But even still, those who use RSS as a way to follow news are the exception, not the norm. And since you can follow most news sites via Facebook and Twitter, RSS isn’t getting any more popular.
Not to mention, of all those who do use RSS, how many use Mail or Safari to follow their RSS feeds? I don’t know a single person who does.
Despite my self-professed “techie” status, I’m ashamed to admit checking my RSS feeds is my Achilles heel. I hardly ever do it, and I know I should. The problem, which Shawn alludes to, is that the majority of my news-catching is done via Twitter. Not only that, but often I’ll tap on a link to a story or blog post within Tweetbot, and skim it in the browser.1 Then I’ll send it to Instapaper for reading later2, therefore Twitter acts as my de facto RSS client. It’s a shame, too, because I have Reeder installed on my iPhone, iPad, and both my Macs, but I hardly ever launch it. It just sits there in my News folder, right next to Instapaper, needlessly neglected. And when I do launch it, it's got >1000 unread stories for me to mark as "read".
As for RSS in Mail and Safari, it’s never made sense to me that Apple included it in the Mail app of all places. Count me as one of those people who never check their feeds in anything but a third-party app.