This practice really is not much different than restaurant and store owners, managers and employees using Yelp to rate their own businesses higher, and give all of their competition bad ratings. You can spot the trends, such as when a Yelp user gives ratings to a lot of similar businesses in the same day, or the complaints against competitors seem vague or generic. I would not be surprised if some of these same plugin and theme developers who leave themselves five star ratings, visit competing plugins to vote them down.