Hey! Whats funny is that I know where you got this essay prompt because it was the first practice essay prompt I used! Anyways. Here is my opinion on your essay (you are very brave by the way).
Rating:
3 to 3.5
Reason:
First off, I agree with PeakPerformance. I got a 4.5 on my first GRE (I took this one without any practice) and I didn't pay much attention to how many paragraphs I used. Its FAR more important to show them how well you can analyze a question and organize your response. Don't mince words, just cut to the chase. Use this opportunity to show off your understanding of the question and its implications. Discuss the topic it suggests, offering your own opinions, and then follow it all up by attacking the question itself. Generally all of the real GRE questions have varying degrees of weakness.
ALSO don't fall for the trap that there is one correct position. Typically the GRE sets up the question so that it can be decently well defended regardless your opinion. You can readily assume that there will be more than one correct opinion and as such you should discuss the merits of both. This is mainly why your essay falls into the 3 to 3.5 category. You discuss one side for the most part.
Your overall writing style is fine and following this advice your should get a 4 to 4.5 with minimal practice.