Art, the reason for drawing and leaving geometry on Layer 0 is because if you don't, it is very easy to mess up your model. Unlike many applications, SketchUp layers do absolutely nothing to separate entities from each other. They are only used to control visibility. If you inadvertently place some geometry in a component on a different layer than the rest of the geometry, it will easily be screwed up.
I've repaired a number of models for others who didn't follow the "rule" about handling layers. The last one I did was a very complex desk. When you turned off the layer containing the drawers, several faces in the case also disappeared leaving openings in components. Parts of moldings would disappear when you hid other layers with the intent of making the moldings easier to see.
Play with the layer visibility on the attached SKP. This was purposely done and is very simple but I think you can see what happens when a few edges are placed on a layer other than Layer 0.
Besides, it's a whole lot less fuss to just do as they say and leave layer 0 as active. There's really no reason to put geometry on other layers and you're less likely to mess up your model. Associating layers with components/groups is sufficient for hiding them when you don't want to see those components/groups.
Think of layers as being the control for the cloaking device. When you turn off visibility for a component it hides the geometry.
Irony: The opposite of Wrinkly