I have to admit they are useful in my opinion. Say you have 10 blogs that you do SEO for … you can use the RSS feed with an IFTTT Applet to auto generator your social media announcements of new content. Rather than posting something to Facebook, Twitter, LinkedIn, Blogger, Tumblr, Instagram and so on and so on.
You can also ensure that your blog regularly gets updates which improve the likely hood of crawlers visiting your site more frequently. It’s not the main attraction content that you would be pulling in but filler content that is still relevant. Think of it like this … when you go to your local newspaper’s site likely all of the nation news is from the Associated Press. That means that article is duplicate content but duplicate content isn’t as bad as folks make it out to be…it happens and in journalism it’s more the norm than the exception.
Sometimes I feel that RSS content aggregated well can create unique content. An example is during an election I built a site that aimed to have all the local elections news in one spot. VoteLouisville.com pulled the RSS feeds of anyone running for office which created a place where you could read two candidates updates side by side and compare what they were focusing on in the election. The candidates want there content reposted, and the voters likely want to see both sides so in that scenario I think the content from RSS made uniquely different aggregated content.