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.
Health applications - a review for an app which allows you to record you stool consistence (for Morbus Crohn) could negatively affect your chances for employment as well as open you to ridicule by others.
"You gave AcmeFamilyPlanner 4/5 stars, Mrs Candidate - say, do you want to have a career or a family?" (Hm, of course this could help remove bad employers from your list of prospective work places ;) )
Politics app. Sex app. Porn app. App about guns. Violent game app. Match.com or OKCupid app.
Pretend you're a school teacher with a few malicious students that have access to Google. This exposes to them what you have privately on your phone, so you're no longer going to rate apps in the ecosystem.
I have one because a lot of tech people are on there and it's easy to get news I don't wanna subscribe to. But given that Google won't allow me to use my actual internet identity of ten years, I've merely got the account set up as a first name and an indistinguishable initial. Google have done a fantastic job in alienating me as a user. What's hilarious is that the name I want to use and wouldn't mind associating with my G+ reviews is the same name that I use on my email account. Google apparently doesn't care though, so no reviews from me either.