Doesn't Google automatically create a Google+ account for everyone who has a Gmail account? It uses all the same details and data, like mjschmidt has said, there is no requirement to actually use g+, essentially, when leaving a review, all it is doing is linking that review to your current account that you are logged in on?
If you want complete annonominoty(sp?) Create a new account, and download new apps from it, to leave feedback. It will mean you have to add two accounts every time you reset or change phones etc, but if that's what you want!
It's not like Google force you to pay for things, you don't like it, you don't use it....if you wanna leave the developer feedback, email them!?