It's not so new, I've seen this couple of weeks ago.

relcontent was added yesterday, Feb 28th at 8:13PM CST

http://72.14.207.104/search?q=cache:-seuAg8sqU8J:base.google.com/robots.txt+&hl=en&ct=clnk&cd=1

that cache was from Feb 23, less than a week ago

what you might be thinking of is relpage... it has been there for as long as I can remember.

Is it benefitical? It does send visitors away from one's site.

Sending visitors away is one of the main job of Google, blogs, digg... :)