Absolutely "pro".
Without a union I'd be making half the wages, have no benefits, and be at the mercy of the next insane boss that wanted to fire me for the halibut. Without a contract, a company can too easily get out of its agreements to provide benefits, or reduce wages.
That doens't mean that all unions are created equal, that they are all effective or have good leadership or bargaining skills.
But: remember, when you are in a union, YOU ARE THE UNION. Just as with any other community, you choose whether to participate, make it better, speak up.
With a labor contract, you reduce arbitrary treatment and can have clear guidelines in the workplace. That works to everyone's benefit.