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.
My Google+ network doesn't have much activity relative to Twitter/FB but I still visit it once a day because the content is mostly unique, not the same ones I see on my Facebook and Twitter streams.
For me, G+ is where I post things I find interesting, usually links and thoughts to a relatively limited audience of real-life friends and acquaintances, and sometimes tech-related thoughts to my Hackers circle.
At this point I only use FB for a) chatting with people whose phone number / email address I don't have, and b) to keep in touch with and get updates from more distant family/friends (e.g. "that waitress I worked with a few years ago got engaged-- best wishes!", "it's my buddy's college roommate's birthday-- let's go get a beer in Portland sometime this summer!")
And I only use Twitter to read jokes from wannabe comedians and publicly bitch about corporations...
Basically, I don't want to get a link from the author to her own post, I'd rather get a link from someone who read her post or article and found it interesting. I realize it doesn't involve a sexy machine learning algorithm, but it sure does work well for me.
The twitter-clones tried this approach, encouraging content-"seeding" with a full r/w API. What actually happened is that they (nearly?) all ended up as networks full of bots spamming each other with duplicate content.
And it's hard to sell advertising when your only users are robots.
So whether it took Facebook an hour, a decade or 2.5 years to roll out their API, right now they've got one and right now google is trying to compete with Facebook through their g+ offering.
That must be why ads for G+ are never ever about sharing little silly things with your friends... no wait, they're actually trying their hardest. It's just adoption sucks, so people rationalize it. That's how I see it, anyway.
I'm following the majority of my family and friends on google plus (yes, wow, they all have accounts) and not following any of the I'm-using-this-as-a-blog accounts like hacker news, google developers, etc.
When I login, what do I see?
"Nov 15, 2012 - Shared from +1 - Public"
The previous one?
"Oct 12, 2012 - Google Mail - Public"
So I mean, it's not like no one is using it at all, but no one I care about updates from is using it.
Family-and-friends are on FB already, so using G+ for the same thing will be disappointing for years unless you have a very active set of friends. I keep FB private, G+ public. I have a few Twitter accounts for different apps, but I mostly consume lightly.