Absolutely they should unionize. 90% of these guys are there to provide teammates and opponents for the other 10%. They work 29 days a month for seven months, making slave wages, and are dismissed en masse making room for the next crop.
Meanwhile MLB can buy out The Expos for $150 million or loan teams $20 million at a time.
I'm not buying the "They don't have a gun to their heads" argument for being a non-union shop. The MLB guys pre-Curt Flood had no gun to their heads, either. Hockey has a minor league system that actually pays players to play. There is no reason baseball can't do the same.