I nearly joined a union once. It happened like this.
In the 1980s, I was writing for several magazines, on a freelance basis. I was one of the few people in the UK who could A) write and B) understood PCs, so I was in considerable demand. I could phone the editor of one of several magazines, suggest an article, agree length and deadline and I was all set. And I could knock out a 1000 word article in about an hour, earning about £80. I would write this on my laptop NEC 8701a computer during my commute, squirt it to a PC, spell check and tidy it up, print it out double-spaced (that's what they wanted) and post it off.
After doing this for a few years, I was very familiar with all the editors and staff of the PC magazines, and I would often visit them and blag items of hardware, manuals, software and magazines. And it was fun.
So one day, as I was on a raid to "PC Magazine", the editor, Tim Ring suggested that as I was writing more for the magazines than most professional journalists, I should join the union. So I did. Or at least, I tried to.
I sent in my application, being honest about the fact that I was doing this part time. And I was rejected, on the grounds that I was part time. I told Tim, and he said, "Oh well." And that was that.
Until the Great VNU Journalist strike. Tim asked me if I was striking also, and I explained to him that I certainly would have, except that the union had rejected my membership application, so I couldn't strike officially. Although I told him that in solidarity with the VNU journalists, I would only write for the other publishing houses until the strike was over.
Which wasn't much of a sacrifice, because with the editor on strike, the VNU titles weren't appearing anyway.
Anyway - that's how I know that being part of a union gives you more power than trying to strike deals from a position of weakness.