Most of my family members will sooner dump Gmail than create a Google+ account.
None of the people I follow on G+ are 'friends' with the exception of co-workers, instead, they are people who have very interesting things to say and who I like interacting with. Twitter would be a comparable use case, but it is too annoying to have long form stream discussions with followers like this.
Having a segmented by interest social network is a primary benefit. I come to HackerNews not because I'm friends with anyone here, but because I can interact with interesting people here. It's also why I go to G+ every. The level of dialogue on G+ is much better than Facebook on average, and the G+ communities are a lot better than Facebook pages.