> Unions make sense if the employers have the ability to exploit the workers.
This is an overly narrow definition, so I label it is largely incorrect.
Think of it this way: society involves dynamically-shifting power balances. To the extent the goals of a society include:
(a) standardizing certain rights and norms
(b) providing checks and balances so the powerful have less temptation and ability to exploit the less
We don't want to rely on only one mechanism to achieve (a) and (b). Sure, we want healthy markets and skilled workers. We also want workers who have enough flexibility to change industries and to move geographically. But we also want effective regulation. And we ALSO want organizations that can directly advocate on behalf of workers.
> So unions are of little use anymore, except for perhaps using the power they have to exploit employers.
This statement is silly and ignores the realities I've seen. Sure, a union left to its own devices might be tempted to push too far. But given the dynamic one tends to see in western economies, there is always some kind of negotiation with the associated corporation. Such negotiation is quite similar to parties in the legal system hashing it out. Both may start off far apart but the negotiation tends to lead towards compromise. In the United States, unions that push to far face public backlash and even Presidential action.
I'm not making any dogmatic claims here. I'm simple rejecting overly simplistic arguments against unions. I will readily accept that unions are not silver bullets and there are documented cases of corruption and misaligned incentives. But the solution is not to argue that the notion of a union is useless. The intellectually honest solution is to seek achievable solution in the mess that is the real world. I laugh at people who aren't even trained economists who interpret their models as some kind of reality -- or worse, some kind of normative specification.
Very few dogmatic claims pro- or anti- union survive contact with reality.