Here's my two cents. In order for electric current to flow it must be drawn by a load, which is like a hungry energy magnet. The resistance in Ohms is a measure of the load/hunger. The higher the resistance in Ohms the less energy flows. Conversely, as the resistance of the load drops, the magnet gets bigger, hungrier for more energy. Good amps can accomodate these load variations and spill more juice into the load. But as the resistance of the load approaches zero the amp goes nuts trying to fill the void and hopefully overheats and shuts down. Any speaker's ability to convert this energy into sound at a given frequency varies, so I wouldn't say offhand that the rating in Ohms will give you much of a clue as to efficiency. The rating does point towards the adequacy of the amp you'll require to feed it. The bottom line is that amp designers and speaker makers have to work from some kind of starting point to get these components to work together, which for some reason has a baseline of 8 ohms. Is it arbitrary? It's probably just a freak of nature stemming from the measurement of early design efforts that later became an agreed upon average design target.
