Uneven tread wear, especially front tires vs. rear tires, will give you uneven braking performance and poor handling- especially under adverse weather conditions!
Consistent tire rotation ensures even tread wear. In addition to the better handling and performance, even tread wear keeps you from replacing tires one- or two-at-a-time. Replacing tires individually keeps your tires in a constant state of uneven tread wear.
Balancing your tires reduces the wheel's vibrations, maximizing your tires' expected service life.