why low impedance speakers?

I have heard it's a specmanship game. I 4 ohm speaker will be louder at 1 watt at 1 meter than a similar 8 ohm speaker. Some speaker designers feel pushed by the market to design at 4 ohms so they can compete in this spec game. I know to most A-goners that sounds pretty lame--we're more interested in what a speaker sounds like--not what specs it has--but more than a few speaker manufacturers have illuded to this as being the reason.

Mborner is correct that a 90dB speaker is just that 90dB at 1 watt 1 meter. But that's not really what I was getting at. It is a combination of his last statement, but more importantly the speaker manufacturer/designer can implement a design on a 4 ohm load that will result in higher output for 1 watt versus a similar design at 8 ohms. So what I am saying is that if the requirement is to design a speaker that has 90db sensitivity--it's easier to do with a 4 ohm design. It is in part a specmanship game. What is important is that if it is a really low impedance--dips into the 1 or 2 ohms--you have an amp to handle it. Most amps have no problems with 4 ohm loads (but again, that's only a nominal load).