The value they add is easily measurable, but it's still quite difficult to tell whether their performance is due to individual merit or external factors.
For example, in any given year some traders are wildly successful (and reap huge bonuses) but year to year their performance is variable enough to be noise - it's not clear that really good traders really exist. Some are just bound to get lucky.
Similarly sales guys may appear great because their product sells itself, or terrible because of a downturn in the business cycle or sector.
It's not insane to want a read on "how hard are you working" or "how skilled are you" that is independent of all the messy confounding variables of the market.
