First, G+ has three values for Google. 1) It gives them rich information about their visitors demographics, interests, hobbies, friends, etc. 2) It's a platform for displaying ads. 3) It's a way of screwing with Facebook, who is a potential threat.
But as Facebook has learned, social networking websites are very poor places to display ads. When someone is searching for a "web host" or "how to stop mysql from crashing", a targeted ad for a new web host or a new database engine is (relatively) likely to be clicked on. When someone is checking to see if their friend Hazel has posted any pictures from the party last weekend, there is no targeted ad which is likely to be clicked on.
G+ is simply not a good platform for displaying ads. And trying to do so too forcefully will push people away, which will undermine reason 3. It will also - crucially, from Google's point of view, since it's (probably) the entire reason they developed G+ - it will undermine reason 1. Google's core business isn't throwing ads at people, it's throwing carefully targeted ads at people, and G+ is a major part of their strategy for getting the data they need to improve their targeting.
The Google+ API is not designed to empower third-party developers. Maybe they're still searching for a way to do it right, but currently it's too limited for an ecosystem to flourish. Why should a challenger come from a big company anyway? Both Facebook and Twitter were small startups during the fall of Myspace.
Facebook + Twitter alienate their users by watering down the experience with ads, losing the 'cool'ness they need to keep eyeballs on them.
I think that's where Google+ can swoop in and make Twitter+Facebook look like Myspace. Thoughts?