I have abandoned RSS a long long time ago and I know Instapaper is not really meant for that. Instapaper does have an email address you can send stuff to, so you could probably automate (IFTT?) an RSS feed to send email to that address.
For me it's a "read later" app. I noticed that I enjoy sitting down and reading a bunch of articles distraction-free, but the various sources that I get my articles from (Hubski excluded) are optimized to keep you scrolling, not to have you read stuff. So I now shoot articles I read on Twitter off to Instapaper so that I can actually read it with focus instead of reading about it half-distracted by the next shiny thing.
Plus, its highlighting function is just perfect for me, as that's what I always want to do with text (but rarely get the chance to). I can now highlight interesting passeges and easily send them off to my notes system in Google Keep after finishing an article.