I don't think GP is saying that the demand you add to the economy increases linearly with income. I think the point is that it's essential to have lots of people with a healthy amount of discretionary income, because non-discretionary income can't really be reallocated no matter how compelling your wares are.
Yea, it's more important because there are more (or should be more in this era) people in that range as a percentage of population than strikingly wealthy people.
