My second stint at UC Berkeley was as a postdoc where, like all other postdocs and many other staff positions, I was actually represented by the UAW. This was in the physical sciences, so not the kind of crowd that would join the picket line alongside Humanities. The UAW was entitled to a small fraction of my salary, even if I wasn't a member, but my health insurance was badass.
"Hi, I'm Todd. Your UAW representative. Are you interested in becoming a member of the union? We have monthly meetings and your monthly membership dues would be XX."
"No. I'm too busy."
"I understand. Are you aware that xx of your monthly pay goes to supporting us regardless of your membership?"
"Yeah. I'm cool with that. Keep up the good work. But really, I'm too busy."
My experience may be unique, having worked in Libtown, USA. Now, as a researcher in Corporate America with a family who thinks my healthcare premiums and deductibles are too damn high, I'd be down with union membership. Still, I think the incubator of the concept is earlier in the professional career, i.e. in academic environments, and those folks are too overworked to care, and too beholden to unsympathetic faculty.
/csb