DISCLAIMER: I'm not a Google employee. This is my guess.
Users doesn't make money for Google. Users are valuable only when they're in sellable state. Ad companies don't want to buy fully anonymous, bogus or false profiles. They want real personal information from real human. To increase efficiency of target reaching / marketing cost.
Until now, Google didn't need to worry about quality of their product - I mean their users profiles. Because that was valuable enough. Because nobody else could provided anything better.
But now it's a little bit different. Especially with Facebook. Facebook provides better product - more accurate, proven, related, real personal information. There's mostly no spamming, bogus, false account. They offer interconnected and very clearly tasted profiles. The most fantastic thing is all the informations are input by users themselves. So accuracy of the data is incredible. Google' product - user profile - is mostly tracked by usage history. So inaccurate. People refuse to input something on Google. This degrades quality of their product.
To the marketers, Google product - user profiles - are now inferior. Nobody wants to buy Google product anymore. Not completely useless yet, but it's not competitive product to what Facebook provides. If this situation continues, Google has to bargain a lot, and finally will lose the only their profitable business - ad selling. Ad is not just an empty space on a website. Nowadays, you can't sell ad in high price without targeting information. Google's targeting was best in old days, but now Facebook offers even better which makes Google product crap.
That's why Google is pushing everybody to their copy of Facebook - G+. To survive. By making money. G+ can make money by delivering quality user profiles to marketers. But you, the users, are just nothing if you don't offer that informations. Because without informations, your account is just an useless binary junk which can't make money.
If you still love Google, please, feed them your personal information. That's the only way you can keep them (and their services) to survive. Anyway don't forget that any further marketing junks are also your responsibility. That's what you pay for Google stuff.
P.S.
If you think something wrong in my posting, please correct me. I also want to know if there's any other reason.