|Credit cinemapolitica.org / "Norma Rae"|
It seems to me that actually becoming a member of the union (as opposed to simply donating the dues money) is a fairly serious issue: it says that when your brothers and sisters call for a job action (WRONG here, see below) because of a disagreement with management, you'll join them.
I'm not sure that's a complication that I'd want during a postdoc. But it's Sam's choice, and I'm sure he'll make a good one. (Lame ending. -ed. I know, but that's how I really feel.)
*I prefer Billy Bragg myself (ironically).
UPDATE: Jim Austin of Science Careers posts a much smarter, more relevant comment than my own:
I'm agnostic on the issue myself, but you should be aware that there's much more to unions that job actions. In fact, many contracts -- including the one just agreed to by the UC system and their postdocs, puts strikes off-limits.
If you want something to be concerned about, you should (IMO) be concerned about the implicit divide this suggests between management (the PI) and the workers (the postdocs). It would be far better if they could be completely on the same side. But as job prospects in academia deteriorate and the employer-employee relationship starts to seem more exploitive, that kind of relationship becomes harder to sustain.
And anyway, there's no reason why the PIs should not be on the side of the postdoc unions. Sure, there's some financial incentive, since postdocs get paid out of grants. But many PIs want what's best for their postdocs anyway, and many seem to support unionization. (This is anecdotal; I haven't seen any studies.) I guess the bottom line is whether being involved in union activities will hurt your career prospects. So far, I haven't seen any evidence that it does.