Here is a simple explanation as to why tire rotation is a maintenance item worth keeping up with.
The simple fact is... your front tires wear faster than your rears. No mater what car, no matter how well aligned, the fronts will always wear faster than the rears. This happens for two reasons.
1. The front tires do all the steering while the rear tires just follow. Think about all those tight parking spots we've maneuvered into and all the three (to six+) point turns we've made. The front tires are twisting against the pavement while the rear tires are (for the most part) just sitting there waiting for us to figure out the fine art of parallel parking. That twisting is wearing down the rubber of the tire just like rubbing an eraser across paper wears the eraser down, only much slower.
2. The front tires are rarely ever sitting flat on the pavement when you are driving. Weather your vehicle has independent front suspension or a solid front axle, when the vehicle is in motion and you are making steering adjustments, the design of the front suspension causes the front tires to contact the pavement off center, which over time will wear down the outside edges of your tires.
(Worn front suspension components or a vehicle that is out of alignment will also cause uneven tire wear. As will improper tire pressures).
Rotating your tires helps spread the wear from the front suspension evenly between all the tires. This will help get as much life out of a full set of tires as possible. Also, rotating your tires a few times a year will help prevent the wheels from seizing on, which can make a roadside flat tire change worse than it should be.