Depends on how you want to read it I suppose. I don't think Gene is trying to imply that any old amplifier will handle a difficult load, simply that an amplifier with a stout power supply will hold voltage steady regardless of impedance.
I'm not even going to pretend to understand that.
Lets put it this way: suppose an amplifier can "double down" into a 4 ohm load from an 8 ohm load. That means into a 4 ohm load it is delivering doubling the current for the same amount of voltage relative to the 8 ohm load. Therefore at the 8 ohm load, the speaker isn't current limited, but voltage limited.
You may not think in those terms, but someone else might. And there's nothing wrong with that.
Depends on the level of price and performance we're talking about to some degree I suppose. On the mid to higher end of the scale the price of high performance amplification will tend to be dwarfed by the price of a high performance speaker. An Emo XPA-5 for example can deliver solid output into 5 channels for $180 a channel (and less on sale). A pair of Ascend Sierra Towers OTOH will run $950+ a pop. In the grand scheme of things here, the amplification is a pittance, and limiting yourself wouldn't be advisable IMO.
Of course, if we're talking about lower end gear, yeah someone whose got $200 to spend on speakers and not a dime more with a lower end receiver should probably avoid a 4 ohm rated model with dips down to 2 ohms.