although above... (transition level) ...everybody should be at a quadrantal level and therefore vertically separated.
Indeed, above TA, in class G, IFR you must fly quadrantals, but we're kidding ourselves if we think that's giving us much separation from other traffic. It forces all IFR aircraft in the cruise OCAS to fly at specific levels, concentrating them into layers, just because they are following vaguely similar tracks doesn't help, because as IO540 said, there can big speed differences, also two aircraft at the same QL can be on converging tracks at right angles.
It might be better to just let all the aircraft spread out to whatever level they like and rely on the big sky principle to give separation.
