Over the course of this nations history Unions have served a great benefit to many people. Also keep in mind that they were most important before you had allot of the labor laws that are on the books today that encompass many of the rules that previously were union based.
As for Wallmart, you're right, they are not a union based organization. They are also a great benefit to millions of americans who are able to purchase things there at a greatly reduced prices, thereby saving themselves money. If they were to unionize, and increase their prices accordingly, do you think people would buy as much there as they are able to now? Would that be good for the general public and the economy, or just the members of the union?
Look at the UAW situation today. Great Union right? Perhaps, but now the big three have laid off tens of thousands of workers yet still continue to lose money. Part of that is product related, but part of it is because companies like GM have to make $1,500 on every vehicle sold just to cover the costs of benefits to it's workers.
I work for a RE developer and we build hi-rise in Manhattan. All of our work is done by the Unions and we want it that way, so long as they don't get so out of control that it no longer makes sense. As hard as some of the trades work, there are still fluff-jobs in the system that get a little crazy. That is one of the reasons that some developers have gone non-union. If you are going to build and be driven nuts, you may as well make more money in the process.
Unions have there place, certainly not what they used to be, but until they stop protecting the slugs who may be among their members, they will be hard-pressed to overcome the stagma many of them have.