Is it ok to use cruise control in the city?

I use it all the time, I love it. My mom was driving with me and told me that it wasn't safe to use it all the time. How is it unsafe? I don't have to look at the speedometer every 2 seconds, the cancel button is right there (I use it a lot), It saves money (better MPG, and Tickets, school zones). I am more focused, since I don't have to worry about how hard to push the pedal, and I don't have to look at the speedometer every 2 seconds to make adjustments. I wouldn't use it in traffic or rain, I set it to speed limit and hit cancel when I see something I am unsure of (people, kids etc.)..