In all the discussions I've ever had in my non-union shop, I've made the same basic argument:
Unions nullify the company's control over productivity. Reduced output makes the company less competitive, and the company goes the way of the dodo.
Union, by definition, means being yoked with 'like-minded' people who share a common goal. I would have NEVER supported a union movement as I have never been like-minded with those who would. Their goals and my goals never aligned.