I am glad most places are getting rid of non-competes. But here is the best argument I've heard for them:

For many companies, a lot of their value is in their intellectual property. Non-competes exist not because the company will enforce it against employees (they might, but they usually don't), but more as a fig-leaf to potential investors down the line asking about the value of the intellectual property. The argument goes, if someone could easily leave the company with the knowledge earned and go to a competitor, then the investment wouldn't be as valuable.