Completely OT: I find it interesting that this post and several other HN posts this week are hosted on Google Plus. I definitely would not have predicted that G+ would encroach on the LiveJournal/Tumblr space.
They are important as additional clue to users.
They are important for SEO.
But most likely it would take Google+ years before they implement it (like it was with Blogger).
I very much doubt that users care about urls. (Yeah, you care, I care, sure, but nobody else.)