I think that in an ideal world, it is more effective to refrain from multi-tasking and instead to focus on an influx of information provided by a skilled teacher. Unfortunately, many classrooms do not represent the ideal world. Teachers may be unengaging, uninformed, or just plain not good. Students may be unprepared, overqualified, or uninterested. Sometimes there is simply a mismatch in expectations and understanding. If the professor and students are driven to overcome these inefficiencies, and work incredibly hard to avoid them, then and only then do I think that this "rule" should be put forth. Although, one could argue that students and professors should strive to achieve this goal in every setting.