The other problem is that there are a dozen or more cab companies in SF, so sometimes you'd call a cab from one company and see empty cars from other companies

The other problem is that there are a dozen or more cab companies in SF, so sometimes you'd call a cab from one company and see empty cars from other companies drive by. So you have to make the choice between alienating the company you called by taking a competing cab, or waiting and hoping that your called cab shows up.