Many students that I have taught have regarded writing an essay plan as a waste of time. The only answer I can give is that it depends on the individual. If you are a good essay writer who can automatically organize your ideas and structure in your head so well that you can produce a good structured essay without planning, then I say that's it's fine not to write an essay plan.
Also if you're really short of time and you need to get writing on page, then you don't want to waste time on planning. However, if none of these conditions apply, then 1 or 2 minutes thinking about your ideas and how you are going to present them will not be wasted. I'm not saying that you should spend 10 minutes on this. Just take a scrap of paper and jot down some ideas that you are going to use in your essay.
Then you can divide the ideas into 3 or 4 paragraphs in a logical order. This shouldn't take you long and the structure that this will give your essay will be well worth the time that you spend doing it.
