Now, I can understand why you may view essays as valid rebuttals, maybe even THE valid rebuttal. They are hard for your adversary to read through, and can cause emotional damage that will cause them to drop the argument. You can put lots of information about your case and why your opponent's case is stupid. However, in this essay I will state my case on why essays are not good rebuttals.
First of all, essays take a long time to type. If you just spew out random words, your essay can contain information that your opponent may use against you if they actually read the whole essay. You must think carefully about each and every sentence and word that you type in your essay. This is time consuming, and damaging to your argument because it gives the opponent time to come up with a good response. Even when you have everything down, it still takes as many as 5 to 10 minutes to write everything down. So, perhaps the length alone is too much. But, if this does not seem an issue to you, I will now present my second reason, overcompensation.
What I call "overcompensation" is putting too much information into one response. Information the opponent might use against you. This is not a good idea, as you are firing shots that might miss, and your adversary can fire back at you. Therefore, overcompensation can put your argument at risk. Of course, now I move on to my third topic.
When you type out your essay and click "Post comment", odds are you haven't really thought too much into what exactly you put down. As I have stated in my previous arguments, this means your opponent can easily take your foundations, which while appearing large is nothing more than a hollow shell, and break it down, and use the pieces to construct a stronger foundation for their own argument. You may have forgotten what your counter-arguments would be, or perhaps you haven't even prepared any counter-argument because you were too confident that the opponent would be afraid of your illusion of a strong foundation and back away. But no, most will see right through it. Therefore, it is a very, very bad idea to make an essay with little thought.
To conclude, I will sum up everything I have said. Essays are time consuming. Not only that, but you often condense too much information into one argument, allowing the opponent to pick you apart. Finally, you likely haven't thought about what you are putting in your essay. An essay is only a hollow shell, and I've run out of space.