It's not replacing RSS, it's still an xml feed, like it always has been. It's just adding a redundant delivery mechanism. If your http server goes down, how does anyone get your feed? Wouldn't it be nice if there were other servers with your feed an app could go to get your feed in case your server went down. And your server would still send the feed over http like it always has. Doesn't replace anything, just gives you an additional means of distribution.