Why low sensitivity speakers?

Perhaps my explaination was not clear. It is not the driver response, but rather the crossover that makes the efficiency (thus sensitivity) change. Gs556 analogy is really excelent and as to how it applies to speakers, it's back to what I said in the beginning. Lower frequencies require more movement of air to achieve the same SPL, thus they are the "heavier bottles". Great analogy Gs5556.

In response to bass: a stiffer cone (woofer) is required to produce a 'tight' bass. a cone that is less stiff will deform, causing muddy, poorly reproduced bass. the tradeoff here is that the stiffer cone requires more power (is less efficient).

Karls, I'm no expert and you seem to have a much better understanding of these things than I do. However, it would appear to me that horns don't see more air in fact I would think they see less air. I'm guessing that the horn compresses the moving air so that there is less lossy disipation and therefore better efficiency.

Pmwoodward - Why would a stiffer cone be less efficient? A more massive cone, yes. But a more flexible cone seems to me as if it would just do a poorer job of moving the air, thus requiring more amp power to produce the same volume. Isn't this why the ultimate theoretical driver surface would be both infinitely stiff and yet infinitely light (impossible in reality)?