Financially: I am good at my job, work hard, and take on a lot of responsibility outside my formal JD, and it has advanced my career (and pay) dramatically faster than I could have advanced by working within a rigidly defined role with strict pay bands.

Ethically: I do not want a committee of mediocre coworkers telling me what responsibilities I am allowed to do 8 hours a day.

I would feel differently if I was a janitor, of course. So janitors should unionize! But I am a well-compensated software engineer and I have no interest in that employment structure.