How about this explanation...16 ohm low pass crossover coils are HUGE, well twice the size of their 8 ohms counterparts anyway. Plus amps deliver power better into 8 ohms than 16.
On the high pass side, crossover caps for 16 ohms drivers are half the size of what an 8 ohm driver would require. So by using an 8 ohm woofer, and 16 ohm tweeter, you invest less $$$ in passive components. Samyes, i made it up just now, but it makes sense, and MIGHT even be true:)