There's also a risk of the airline going bankrupt completely and having the miles completely lose their value. I have a friend that saved his miles for a long time for a trip to Hawaii, but the airline folded and he ended up with nothing. If you don't plan to use the miles, what is the point?

On the other hand, I like to carry enough of a balance so that I can take advantage of a trip that is a better deal with miles. Some trips are very expensive, but can be had with a low level frequent flyer award ticket. If your account is zeroed out, you don't have the option to buy with miles when you need to.