why low impedance speakers?

The only reason I can think of using low impedence drivers is just what you say to ensure that the owner will use an amp of high quality, to ensure that the speakers as a whole will perform well. I would be interested in talking to someone who has made drivers in the past to see if there are any other benefits.

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.

Many low impedance speakers (1 - 3 ohms) are in the panel family (ribbons and electrostatics). Apogee, Martin-Logan, Quad, and Maggies are probably the most well known. Unlike dynamic drivers (cones and domes), ribbons and panels are low impedance by the nature of their design. Just adding resistance to get the impedance higher degrades the sound quality and dissipates your amp’s power as heat. Using a high quality, high current amplifier is needed to control these drivers and produce the best possible sound.

Low Z speakers (4 ohm) can get up to twice the output power out of the same amp as Higher Z (8+ ohm) designs. They're helping you save some $, spending less, not more.