[galdosdi]

I think you are committing the common engineer's fallacy of examining a situation with Vulcan like logic, reaching a conclusion, and then applying it to a social process.

> A metric that says everyone is under-performing isn't a pretext to dodge accusations of an illegal or otherwise not good pattern of firing anymore than not having the metric at all is.

Just because you (and indeed I) think that way doesn't mean the actual social and institutional actors involved will. In any given hearing for a given matter impacting a single employee the data about all the other employees won't even be available to reach the conclusion you discussed. It would take a much bigger more well funded legal action to push the argument you are making. In fact, how do you even know that US law even allows the argument you are making? It very well may or may not.

It's not a perfect tool, but as I said, it's another tool in the toolbox to muddy the waters and make it more expensive and complex to challenge a dismissal.

I think the way you mentally model socioeconomic and legal processes in the human world is not very true to life. Pure logic is not any sort of big magical trump card. Humans are not rational actors.