I think the imporatnce of rationale can't be underestimated. I think it helps having students "buy into" the subject matter at task. Many times when I teach my class, I encounter subjects that are a little obscure, plain, hard to comprehend, or flat out old school and boring. I personally take a little of the class time (typically when I introduce the subject) to explain the relevance of the subject, and its relation to the overall picture. I normally don't include this explanation in my lesson plan, but I think its a good idea to have it as part of it.