Writing for Macworld, Dan Frakes reports that, amongst other changes, RSS has been omitted from the forthcoming version of Mail for Mountain Lion. Although the omission will likely stoke ever-frustrating cries of the death of RSS once more, I tend to think it makes a lot of sense.
I doubt many people used the feature but, more importantly, it no longer fits with Apple's vision of unobstructed simplicity and consistency. RSS is not a user-friendly feature for the average person. The vast majority of Mail users, I would guess, simply allow the default RSS feeds to idly grow with little attention. Just as iOS foregoes any semblance of built-in RSS, Mountain Lion intends to do the same.
As Gruber rightly argued, Apple is simply trimming the "cruft."
One final thing to note is the interesting juxtaposition between the removal of RSS and the deep inclusion of Twitter. Food for thought, to say the least.