Google Plus Takes Page from Facebook, Shows What Posts Your Friends Like
Facebook already does this by highlighting posts your friends like. On Google+, the person or people who +1'd a particular post that shows up on your feed will be displayed above the highlighted post. In doing so, you're exposed to content that you might have otherwise missed, namely posts that appear on feeds of friends of your friends. You might also end up adding new people to your circles, as well.
This will work in reverse order, too. In other words, if you +1 a post, it will be shared on your friends' feeds, unless you choose to keep your +1's private. In addition, any posts that you've restricted sharing to just your circles will not show up outside of those circles, even if they're +1'd.