I actually think it is a good idea to treat the WC's as having intro/middle/conclusions AS LONG AS YOU ORGANIZE THOSE PARAGRAPHS TO BE APPROPRIATE FOR THE TYPE OF WRITING YOU HAVE TO DO. What I mean is, that if you are asked to write a memo, it's okay to have an "intro"....but your intro should be a memo-style intro which (don't know if you learned how to write memos in college) should present your "conclusion/your decision" first and your conclusion should really just be a little blurb like what vince said "please let me know if you have further questions, "etc. You shouldn't be writing intros/conclusions like how you would for research papers (in my opinion at least!).
On the 2 exams with written communications that I've received scores for (waiting on FAR & BEC didn't have any), that is exactly how I organized my communications and I've scored "stronger" than average on both exams (my state gives out score reports even if you pass). I don't know if this makes a difference, but I also make sure that each "paragraph" is at least 3 sentences long (something I learned in grammar school!) and has correct spelling/grammar. My first paragraph is usually 3-4 sentences, my middle depends on how many paragraphs I need (if I need only 1, it'll be about 6-7 sentences, but if I need 2, it'll be about 4 sentences each) and my conclusion is again 3-4 sentences. I usually fill up the screen and may scroll just a bit. The most important thing I do is check to make sure that I addressed the issue fully. If you need some help on a point or two, you can always use the research tab.
BEC-92(02/2010) REG-90(04/2010) AUD-86(05/2010) FAR-??(08/2010)