We have two notions to consider:
1) The first is what we call, "natural cadence," in the sense that a person is pre-disposed, based on their muscle fiber typing and fitness level, to pedal at a certain speed. The mention of muscle fiber typing leads me to...
2) Specific energy system usage. The lower the cadence, the greater resistance, and for the sake of the analogy we'll call this the same as "weight," on the pedal stroke. We know that as weight against movement increases, demand for muscular "strength" goes up.
This is metabolically important because the more "strength-like" your movement (instead of "endurance-like"), the more cellular metabolism gravitates towards anaerobic cellular energy systems.
Now all of this may sound like a lot of ex-phys mumbo-jumbo, but the take-home point is that amidst a sport that demands sometimes VERY extensive endurance, the more I am encouraging my body to activate anaerobic energy systems (if that doesn't mean anything to you, think quick-action, low-endurance muscle cell activity), the less stamina I will have.
To a certain extent you can train your body to perform MORE aerobically at low cadence, but this is difficult, because ultimately you will always be moving more "weight" at a lower cadence.
Consider this model instead. Take 10 leaping steps with 100lbs on your back, or 15 normal steps with 70lbs on your back. Feel the difference? So does your cadence.
The most biomechanically efficient cadence is 90rpm. In the world of elite cycling, this is actually towards the lower end of the preferred spectrum. We consider below 80 to be biomechanically inefficient, but on some grades of hill, is unavoidable.
Hope that helps!
Spinning them quickly,