Is it cruel to leave a dog outside all the time?

We have a huge fenced in yard and we live in a nice climate. It never snows.. Its almost Christmas and its still in the 70ies & 80ies here. We kind of live in a bad neighborhood and wanted to get like a watch dog. That would be nice to our family & our kids. But if an intruder came around it would bark. We wont get an alarm system because in our city the police department charges $100 everytime your alarm goes off if they are dispatched out. So say we got alot of false alarms we would go broke. I asked earlier what type of dog would be good to stay outside all the time and many people said that was cruel. I know alot of people who keep dogs outside all the time and never have a problem. We dont have a dog, and if its determined that it would be too cruel to leave it outside all the time, depending on what all of you think.. Then we just wont get one.