People who want to see the problems with the underlying concept need look no further than the long-running national ID card debates...
... this is not new and the issues are well known.
This is just in the USA! It seems this is a debate that just keeps going round and round - only the context changes.
Not so say that ID cards are not useful. They are. It's just that a universal, ubiquitous ID system cannot make us more secure. Even if it was perfect. People and organisations implementing any kind of ID verification need to be very clear about what it can and cannot do.
ID verification of some users seems like it would be useful for g+ ... and some people would use it. Enforced for everyone? The harm outweigs the good. Besides - google stands to gain more by allowing a graduated verification system.