Hambone, excellent point.
Consider:
#1) Rear Wheel Weights and/or Fluid Filled Tires, these put weight at the rear of the tractor, but they do nothing to lighten the front end of the tractor. Further, rear wheel weight (in either form) allow both the steering mechanism and the front axle of the tractor to take the full force of pressure of the FEL & its load. Not to say this is completely bad, it is just not optimum for really heavy FEL work.
#2) Heavy overall tractors are typically cast iron machines with cast iron engines, cast iron bell housings, and cast iron rear ends. That means that much of the overall weight is actually in the front of the machine, or at very least spread across the entire machine. These tractors still REQUIRE proper ballast, and there are discussions where even the heavy Mahindra's get their back tires lifted off the ground.
#3) Ballast boxes, as recommended by manufacturers require less overall weight to do the same job as fluid fill because they use the mechanical advantage of geometry (remember Jr. High School math?) and the effort of "lever action" by placing the weight well behind the rear axle, using the rear axle as the fulcrum point to lighten the front end axle so all of the weight of the FEL & its load is not resting on that front axle.
There are lots of ways to be safe when using a FEL, and under modest use rear filled tires or wheel weights are often enough weight. When you are really testing the capacity of your FEL then some form of ballast on the 3pt can make a real difference in safe operation. A H.D. box blade can often weigh close to the weight of a ballast box. Some folks have fashioned added weights onto their box blades. Some people report using their backhoe or their rotary cutter. The common thread is they are using something that is big and heavy, and that must say something.