Brent Simmons points out that according to Macworld’s reviews, RSS support in Mail and Safari are both removed in Mountain Lion.
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.