Unions lost their uselfulness back in the 90's. when they became a glorified middle man.
They are still useful, but I would say they lost perspective and direction, among other things. A large company with many employees almost needs a union to ensure all employees are treated fairly and equally. Collective bargaining can help balance the wants of the employees and the needs of the company.
The problem with many unions is that they now seem to work in the best interests of themselves instead of the workers they're supposed to represent. Unions have become behemoths that, like government, extract more from the employees and return less in order to keep the giant union machine running.
Some think bigger is better; I've seen where small groups of employees certified one major union as their bargaining agent only to find that the union bargained higher wages for the larger group of unskilled labourers in favour of lower wages for the smaller group of licensed, skilled tradesmen, kind of a Marxist thing.
It is not only large corporations that need restructuring, I think it's time some unions were made to start over from scratch with more of an eye on who they represent, and another on reality. I have yet to see a company with a union that didn't deserve one.