Actually, that is the easy answer. Google+ is far from a Facebook or Twitter "answer." I have been on G+ from the start and it is not a social network in the term most of us think. It is a tool for sharing information. Articles, photos, blogs, etc. It is a lot like Google Reader or blogs on steroids. The discussions on G+ are deep and poignant. It is nothing like Facebook. Much more like Twitter as far as sharing information but without the 140 character limit so content is more in-depth.
But this isn't a thread on G+ so to bring it back on topic, no you don't need it to get your GNex to work properly (though Samsung does promote that G+ is fully integrated - whatever that means). I am sure video chat and messaging on the GNex will run through Google+. You also no longer need to sign up for Google+. If you have a Google Account (which every Android user has), you are in and you can circle me