I think it's done for financial reasons. As with FaceBook they need to provide metrics to advertisers before they will put cash on the table.
So what do you do when multiple people have the same name (like John Smith)? Only one of them gets to have an account?
lol LifeInAsia, sometimes the most simple things make us stop in our tracks and say hmmmmm, good point, that post is one.
Say bye-bye to the service. People won't use it online if it's not relatively anonymous. And the only benefit to real names is to Google, not the user - and the users know this.
I once worked on a forum in a professional industry that crushed long established forums. One of the advantages we used was anonymous postings. Even when commenting on professional matters people didn't want their real information used. For example it allowed them to discuss pricing and product design in generic terms, without any of 'Oh, that's Bob from our competitor, handing out advice to Sally at another company'.
Aside from two John Smiths, there's also the problem with celebs. William Shatner had his G+ account shut down on suspicion that someone was registering it as a "fake name". So it appears celebs may have to go through considerable hassle and scrutiny to get a G+ account (not to mention the hassle of exposure). Bad news.
How do they know it's your real name? I am not prepared to believe they make like newspaper editors and telephone you in person to verify that your roommates know you by that name.
Maybe they don't want your real name at all. Maybe they just want users who don't realize they can make up any old name, so long as it's orthographically plausible, and say it's theirs.* There are advantages to restricting membership to people who are dim-witted, trusting, or law-abiding. Or all three.
* My real name, for example, is Zainab Yaqub. Go on. Prove that it isn't.
I forecast a rush on video of old Monty Python episodes... they were full of great names, sometimes with dozens of middle names that took ten minutes to recite.
Dinsdale Hedgehog, aka
Stoatgobbler John Raw Vegetable (whinnying) Arthur Norman Michael
Some screen names are made to fit the allowed characters or easier to pronounce.
My Twitter ID fullscale4me could be a shortening of Fuller Scaleforme from Italy - but it's not. For the curious, its a declaration of preference in one of my niches.