Barbarian cities are very simple. Ancient age barbarians would have no use for workers. Imagine the reaction: "Look at the stupid AI! It hordes all my Workers even though it has no use for them! I can't believe Firaxis made them so dumb that I can get all my Workers back for free!" Now, if they become terrorists (the barbarians, not the disgruntled posters), they could take hostages from the Workers, but probably not enough to make an entire unit (even if they did kidnap the entire unit, the Workers would most definately want to stay away from government contracts for the rest of their lives).
