Well, the closer the exams get, the less essays I'll be writing. Theres just not enough time! I find plans to be useful though... I outline what I would put in each paragraph, make sure I know all the info, and then write out only the paragraphs that sound hard. I also believe that knowing the info is more important than having the practice of writing. For geography essays especially, its more important to jam them full of information (this is what will get you the marks) than it is to have a fluent or sophisticated style. This is true for history and english, but to a lesser extent, as its still the info thats going to get you the marks, but they like some sophistication too, and structure.
In my experience, geo essays are the easiest (mine suck, but I get all the info in!), then english (know a few techniques, a few original insightful comments, keep throwing in question words and you'll be fine), then history are the hardest (you really need to know your stuff). Well now I'm just babbling. Anyway, to answer the question, I think essay plans can be beneficial.