As I understand it, there are many purposes to a non-compete. So they don't go after your clients, so they don't steal company secrets, so they don't even work in the same industry, or within X# of miles of your business.
If you were to hire someone with a masters in horticulture, and after a few years he decides to leave your employment, while you may prevent him from stealing your customers or using your trade secrets (though you'd have to catch him, sue him and prove damages), you can't prevent him from working in horticulture. You can try, and even if he signed a document saying so, the courts (as I understand it) would side with this employee.
But I know they can't put ads in the paper talking about what a horrible employer and person you are. Or he could, but your slander/lible lawsuit would put him in the poorhouse fast.