My point is that a good review (not necessarily a "positive" review) is a good review whether I know who gave it or not. Is a detailed review from an anonymous user giving something 2 stars somehow worth less than a one-liner review from a "known" user giving it 5 stars?

That aside, as greyboy already points out, what is going to stop a competitor from creating a fake Google+ account to give that negative review? This move doesn't prevent that hypothetical from happening in any way.