I am such a person. For most workers, you're right that a union will probably increase wages. But for me, it will do the opposite. In most jobs I have worked, I ended up paid significantly more than counterparts in the same job. I usually was better at my job and put in more hours, so this was warranted. And for promotions, I don't see how this is wrong: most every union of which I'm aware ends using seniority as a factor in promotions, which I generally resent.
I never liked the mandatory nature of unions, either. If they're that good for workers, quit trying to force the whole company to join. I also hate the idea of being forced to strike.
As far as my interests are concerned, a union would end up taking money and promotions from me and giving it to others. So, I will always vote against them. If one shows up at my workplace, I will probably move because someone else offers better terms of employment.