Cannot give the union POV, because I have never worked from it, but sometimes I wish I was union. Unions are, as Lilz mentioned, a great way to scare off the "Right, you are mine now, I will make you dance for your supper" type bosses (experience from other industries). Non-union SM, you have to make sure that your contract is extremely specific - because you do not have the backing of the union's rulebook. Thankfully I cannot dance, because I know a certain company (which I will not mention, but am glad to say has closed) was run by a man who believed that everyone owed him because he employed them, and he was not afraid to make people do tasks well outside of the norm (he proposed I also understudy one of the main actors, which would have involved dancing - see the link to the start of the sentence and my comment before... clever
). In other situations though, I imagine being a union member would be rather limiting, if you enjoy working with small companies/ad hoc performances. I am of the opinion that it all depends on where you want to be down the track - if you want to be on broadway eventually, then union is the only way to go as far as I know. If you want to work fringe - I doubt many of them would be union.