I can't speak about actors, but there was a time when a major benefit of being in the musicians union was the standardization of contracts that might cover comp

I can't speak about actors, but there was a time when a major benefit of being in the musicians union was the standardization of contracts that might cover complicated issues such as credit, royalties, re-publishing, and so forth. So it wasn't necessarily the magnitude of the money, but making it humanly fathomable how the money would actually be distributed. I've heard people say to freelance musicians (like myself), that you don't really need the union until you start working in recording or broadcast. I've not seen a screen actor's contract but I'll bet the provisions for working conditions and rights are a bigger deal than the pay scale. Also, I've read that in general, unionization efforts tend to be triggered by working conditions rather than pay. Even programmers could benefit from that.