It's all about the money, money, money
Beh, la spiegazione, come scrive Dave Winer non può essere che una...
There's a very simple business reason why Google cares if they have your real name. It means it's possible to cross-relate your account with your buying behavior with their partners, who might be banks, retailers, supermarkets, hospitals, airlines. To connect with your use of cell phones that might be running their mobile operating system. To provide identity in a commerce-ready way. And to give them information about what you do on the Internet, without obfuscation of pseudonyms.
That's really what it's about, money. That's why they want you to use their social network, and why they want you to not use Zuck's. Because they want the money.