I was thinking if I should post - because I am clearly not an expert or a speaker designer.
Some things to note though - Actually, speaker impedance (4 ohm, 6 ohm, 8 ohm etc.) - which really is nominal or "average" impedance is mostly a useless spec. That is because there is no industry standard definition of how nominal impedance should be calculated.
Just look at any stereophile chart. Most speakers have impedance and phase curves that swings wildly across the audio spectrum. 6 ohm nominal speakers will often dip down to 4 ohms in certain frequencies.
To my limited knowledge, impedance and power capacity of speakers and sensitivity are design tradeoffs that speaker designers make. The type of driver, the type of crossover, the type of cabinet, the size/volume of cabinet - all factor into this. I've often found that mini-monitors (such as Dynaudio) will have really small cabinets that still do better and deeper bass than most other speakers with larger cabinets. But the price they pay is lower impedance - thus requiring stronger amplification - often high current amps.
Then again, the class of the amp matters too. Many Class D amps for example don't really care much for speaker impedance swings. I'm running out of knowledge here - but I am sure experts like Kanwar can probably better comment on this particular advantage of Class D amps.
A very interesting question is - how much amplification power is really needed? This is interesting because many people run speakers successfully with only a few "pure" watts of amplification, while many even insist that even 100-200 watts is not sufficient to properly service the speaker across the audio spectrum.