I thought it was a sign that a candidate has poor cost-benefit analytical skills.

Only half joking

Hah. The same joke persists in MBA circles, where, as the joke goes, during your time here you learn how to evaluate why MBA degree was not a good idea.

My MBA was free , got me a better job and I still think it was a waste of time.

I want to say that a good chunk of it was a waste for me as well. The network effect, as far as I can tell, was minimal. That said, two classes did make a difference ( fraud in accounting and analytics with R ). Even taking those into account though, I honestly can't say my degree was as valuable as it is sometimes portrayed ( definitely not the asking price ). I sometimes wonder whether it is possible I could have stumbled onto R on my own ( I was already playing with Python at the time ) and would I have the patience to go through sample exercises on my own. It is hard to tell. I can't honestly say it was a complete waste of time.

One of my professors said that half the lack of women in STEM professorships could be explained by female students who are smart enough to earn a PhD instead also being smart enough to earn an MD or DVM instead.

This seems a bit of a broad brush. You can hardly fault someone for getting a PhD because of something like graduating into a recession, because they actually wanted to into academia but changed their mind, or so that they could immigrate.