[libber]

Because google cannot make as much money off of an account named LispNerd3k vs your real name.

From an advertising pov G+ wants to have as much information about you as possible to best target ads. This is basically a good thing as you see more of what you want and they get more clickthrough and can sell ads for more. When you type something into google search, google can make an educated guess about who you are based on your query, geo information and past searches. A primary goal of G+ is to associate the G+ information you are voluntarily filling out (real name, address, hobbies, etc) with the rest of googles ad network making it leaps better.

That is why google thinks social networking is important, and why g+ doesn't need to "win" vs facebook for it to have a net positive effect on google. Even mild G+ adoption makes googles whole network of ads much more intelligent.

I am not a business strategy person but this explanation makes sense to me so take it as a guess/opinion.

[pluies]

Correct me if I'm wrong, but this has nothing to do with the real name vs. pseudonyms issue, right?

If Google knows you as LispNerd3k, living in Philadelphia, about 30yo, geeky, white, male, past searches "xorg issue fedora 15" and "sicp", that's more than enough to give you targeted ads. As long as your pseudonym is consistent across websites (or Google services at least), there's no need for your real name.