I agree about the positioning at various levels. You have different angles at which your bike presents itself at times....steep hills, sharp turns, etc. It makes sense to have light emanating from as many possible viewpoints. Plus, you have varying heights of vehicles.....a semi truck for example.....they may not notice a slightly off kilter and tucked underneath a seat blinkie. I figure if I have one on the seatpost, one on my backpack, and one on my helmet......and then my various movements while riding combined.....then I have a good chance of being seen by almost anyone. It's all about being seen, not rationalizing which place is the best for ONE light......or trying to figure out how one person might notice you over any other.
The other HUGE benefit of multiple lights is redundancy. Esp. on the rear.....on a long commute you may not be aware of a light beginning to fade or go dead. This way you have the best chance of maintaining at least one full power light at all times.