The write API is necessary for blogs to auto-post new articles to G+. Auto-posting blog entries are standard operating procedure for Facebook and Twitter and greatly assist in seeding social networks with content (which users can then comment on and like/+1/favorite).
They way I see it is that G+ is either going for something special in that they would rather see it a ghost town with no spam, than have to deal with lots of content and combat some spam.
I personally love the G+ system, but there is no way that I am going to manually post a blog article to a G+ page every time I publish something.