Probably a special situation, but the Lady and myself have two automobiles between us: a '95 Geo Metro and a '96 Toyota Tacoma. Only one is ever driven at a time, and frequently neither are driven as the both of us are on our bikes. But the two vehicles (which we have discussed the utility of having, as we pay insurance and registration for both, yet only drive one or the other) function as winter and summer vehicles, respectively. And while the Taco has 4WD, the Metro is the winter vehicle half of the time anyway, since it can hold its own on snow and ice as long as there aren't any hills (and you don't drive like an idiot).
Still, after having dropped $1,400 on new suspension springs for the Taco last summer, $300 on a new driver side window on the Metro (punk kids with a rock), and now a 'check engine' light and an unwillingness to start in the Taco, we're seriously considering becoming a one car household and selling both vehicles to buy a Subaru wagon. With both 4WD and a long enough roof to carry sea kayaks, plus decent gas milage (although not nearly as good as the Metro's), it'd span the gap between both of our vehicles. Honestly, if we gave up skiing, kayaking, and backpacking, then we could just give up the car(s). But that ain't gonna happen.
As to why North Americans have abandoned the one car per family model, it's because they can. Gas prices are kept low through subsidies, tax caps, and war. Parking is free or underpriced, and the roads get wider and faster every year. Our society is flush with cars, so you can always find an automobile, no matter your price range. It's just another symptom of consumerism run rampant. It has become a milestone, the age at which you get your own car. Just like the age that you get your own phone, or computer. You are expected to have a car, and if you don't, then you're weird.