I think the problem comes from the fact of treating Google+ like any other social network like Facebook. Google+ is supposed to be an identity layer for all of Google. The fact that they have status update posting, etc is secondary.

Google would like to assign a face, name and phone number to every Google account. And why is that? For various reasons. A face and a name is very valuable for targeted ads. And a phone number associated with every Google account will help Hangouts. Maybe it will enable sending SMS from the web to any phone and also Voice users can cal from the web from their Google account. Thus, Google achieves their aim of disrupting the carriers and making the phone number irrelevant.

That's just the way I see it anyway.