It's a fair question. So is: "What's wrong with monopolies?"
The answer is that both subvert the free market system by compromising individual achievement in the name of "rights" which are not really rights. Trying to justify collective bargaining (especially in the timely context of public sector employees which is the root of the Wisconsin and other protests) or monopolies is like trying to offer 1930s Germany or a popular vote in Afghanistan for imposition of Sharia Law as good examples of democracy.
Collective bargaining is not a right. It is a privilege granted the union movement on a state by state basis. States can revoke the privilege as certainly as they can revoke a driver license from a DUI offender. And now that in the dismal Obama economy the destructive results of years of expanding union rights are apparent, any state that does not work to reverse the trend is foolish.
Collective bargaining is especially toxic in states that do not have right to work laws. We're lucky here in Tennessee that we are a right to work state: no one has to join a union to hold a job. That at least keeps the unions somewhat honest.
But in states like Wisconsin where a union can not only claim the power to bargain for the renumeration and benefits of every single worker in its industry but also can force every single one of the workers in that industry to pay whatever dues it deems necessary, the situation is exactly the same as an economy where monopolies are allowed.
Think Bell before it was busted up as a monopoly, for example. Basically it controlled all telephone service nationwide. Its service stank. Technological advances were slow. Rates were high. And if someone came along with a better idea? Forget it. If it didn't make money for Bell, it was a non-starter.
Same with unions which have collective bargaining power and captive workers. The dynamic turns from what is good for the worker, especially the highly productive worker, to what is good for the union.
And what is good for the union is no better for the taxpayer than a monopoly like Bell was for the telephone customer.