Google+ for Apps has great potential to be used inside companies, as an internal social network. Messages can be propagated easily by people "+"ing other users, and you get a much better way to communicate than email. (Much as Wave intended.)
But unlike many other "social network for the enterprise" offerings, it's porous. You can easily introduce posts to the outside world as well. This is really how most modern enterprises should work. Not a complete sandbox, but a single system where you can communicate publicly, privately, or some combination of both.
I can't see companies adopting such a thing unless they did have access to all users' data. The point is that you use it wearing the hat of your company, not as an individual.
If I was working in a big company, I'd still maintain a separate individual Plus account and use it in the same "semi-work, semi-personal" way I use Twitter. I'd still be using the company account a lot, but mostly for posts and comments constrained to circles inside the company. The only public posts would be more official posts, e.g. new product announcements.