In the UK I pay ~£16/$25 per month to be a member of Prospect, a union for workers in the telecoms sector. When I was made redundant (downsized) they were able to get me an excellent settlement and provided me with incredibly useful professional advice.
I also know that if I'm in a disciplinary situation, they'll have trained employment rights specialists available to fight my corner.
The best way to think about a union (IMO) is as legal insurance. You pay for car insurance in case someone plows into your automobile - why wouldn't you do the same for your career?