It seems that most homeowners do the realize that organic debris on the roof is a defect. In the past, I have written on moss before. However, while easier to deal with than moss, huge quantities of leaves on a roof are a problem too. I have inspected homes where even the ridge vent was packed solid with leaves and needles from trees. A simple understanding of roofs, makes it clear why this is bad. Sloped roofs are designed to shed water. They are not watertight. Even flat roofs, more watertight, should have drainage so the water does not stand in place for long periods of time. When you end up with roofs like those pictured below, the drainage is impeded and, virtually always, the gutters and downspouts are blocked as well. Another issue is that the debris keeps the roof from drying and that is hard on the roofing material, be it wood, asphalt, etc.
Bellingham home inspector
Steven L. Smith