Why do I care how seriously people take my review of a mobile app? I'm just posting it to help others make a decision about whether to buy/download the app. There's not much in it for me either way. If there is any potential downside at all, even a small one, I'm not going to post a review. I suppose some people care about being a top, respected reviewer, but not most.
Can such a competitor not create a "Tom Jones in Skokie, IL" Google+ (along with a myriad of other fake accounts) to achieve the same result? Or, is that to difficult for someone with those intentions?
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.