I agree with what Donald says about when fishing fast water, the fish never really get a good look. I will go a step farther and suggest that Tenkara allows for very precise presentation. When I say presentation, I mean the ability to precisely manipulate a fly. If we limit ourselves to only fishing dead drift, the only thing that separates our fly from a stick or a weed is the fly's profile. Adding movement to the fly is another way to make it look alive, i.e. something good to eat.

In my mind, it's the level of presentation that Tenkara allows that makes it such an effective method.