I agree on that, @good-karma. It could not be in any other way. Most of the times I post reviews which are sincere. And gratefully most of the times developers have thanked me for my reviews and even more times they have fixed problems I said their app had. So I updated my review accordingly.
However, there is a great difference between posting a review on the product listing on the Play Store -or the AppStore or whichever- and asking people to post a positive review which would then be rewarded, no matter if the rewarding comes from the developer or a person who truly loves the app and wants it to become more popular.
Furthermore, a positive review would not make the app improve as it would do a sincere comment.