>>Put simply, the individuals within the quintiles are not static even though the quiltiles definitions themselves are.

By definition, demographic populations are dynamic. Even so, as a term it's not much help to fit young people who want work in communities with unemployment into double figures. To them, claims about dynamism may be a modern version of "let them eat cake".

>>After-tax median income (half of the population is above the median, half is below) grew by 35 percent.

Median is useful for other purposes, but for this purpose all it tells you is that half the population definitely went backwards in their share of economic growth. It doesn't tell you what happened in the top half. Specifically it doesn't tell you that most of the top half also went backwards while the top few grew 275%. This is why the report used quintiles. Just as well, because otherwise interested observers might not realize that the middle class also went backwards... we know that a prospering middle class is a crucial component/aspiration for dynamism as a source of growth and vigor.