I rarely use cabs. If I have an appointment that can't be postponed when the temperatures are below 15 degrees fahrenheit I'll use them. I also use them to transport my dog to and from the veterinarian.
I will use one if I ever must transport something too bulky to carry on the bicycle. The year 2003 was the last time I did that. In 2006 I bought a car. In 2008 I sold it. The cost of cab fare even once a week for short trips is lower than car insurance for one year. I wrote about that in my book "How to Be Debt Free Fast!".