I'm not sure I get your assertion of inertia being better. 6000lbs of truck is plenty of kinetic energy to plow through stuff without additional flywheel effect from the wheels. That rotational and added mass also absorbs energy that could otherwise be translated to acceleration in an uphill climb for example.
A common rule of thumb is that for each 1lb of unsprung mass equals 2x the equivalent of static mass when accelerating on a flat surface. So that 13 lbs/corner translates to (13x4)*2=104 lbs of perceived mass the engine has to accelerate. Now say you want to accelerate going uphill, which now involves accelerating and lifting that that mass of 104 + 52 = 156 lbs. Not so minor for any perceived benefit.
Lighter is truly better.
Rotating mass, by itself, does not absorb energy, provided it is already rotating. Accelerating the mass is what requires energy. The more mass you have the more energy it takes to accelerate to a given speed. But once you are up to speed, the additional energy is stored as kinetic energy and can be applied for useful purposes- like rolling through ditches and going straight through washboard bumps.
Lighter is better for acceleration. If the measure of performance is accelerating up a smooth hill, then the lighter wheels would perform better.
Heavier is better for maintaining velocity. If the measure of performance is maintaining a constant speed over an uneven surface, then the heavy wheel will perform better.
For an exaggerated example- consider riding a bicycle down the middle of a railroad track. The bicycle with the light wheels gets stuck in between the railroad ties- you decelerate every time the wheel hits a railroad tie, and have to expend energy to accelerate and get the wheel unstuck and over the top of the tie.
The bicycle with the heavy wheels maintains a more constant speed and direction; and it does a better job of maintaining velocity through the obstacles. Assuming you are already rolling at a reasonable speed and everything else is equal, the bicycle with the heavy wheels is easier to ride down the railroad tracks.
With regard to wheel weight, it is a trade off between acceleration and intertia. In theory, there must be an ideal wheel weight for every application. Lighter wheels are better for smooth surfaces, like a racetrack (think Lotus Elan). Heavier wheels are better for maintaining velocity (and direction) through obstacles (think Land Cruiser).
Lighter is sometimes better, but not always.
You will certainly get better fuel economy with the light wheels, but you will feel the potholes more and will be more likely to lose control when driving over washboards at high speed.
Back to the alloys.
