Adding a weight box is NOT the same thing as tire ballast.
Weight BEHIND the rear axle takes weight OFF the front axle.
Tire ballast does not. And insufficient ballast BEHIND the rear axle only serves to ADD weight to the front axle if lifting heavy....
Here is an example, I am gonna use nice round numbers...
Say your tractor loader combination weighs 3000#. And thats no weight added anywhere. Lets say this allows the loader to lift 400# then the rears lift into the air. Now you have your 3000# machine + 400# loader load ALL on the front axle.
Now lets add 400# ballast to the rear tires in the form of fluid. Your machine now weighs 3400#, but you have increased what the loader can lift before the rear tires leave the ground. So now you can lift 600# up front. Now you have all 4000# on the rear axle.
Lets add a 400# weight box. So machine now weighs 3800#, and you can now lift 900# up front, but you still dont have enough to max out the loader and the rears still lift. Now you have 4800# all on the front axle.
Lets max it out. Throw a 1000# weight box back there. Now you can max out your loader lift (lets call it 1000#), and the rear tires still stay planted. AND with that 1000# being BEHIND the rear axle, it shifts some of the load from the front to the much beefier rear axle. So you now 4400# machine with 1000# in the loader, may be able to keep 1200# on the rear tires. So you are only subjecting the front axle to 4200#. Alot less that with a weight box that is too small, AND you can lift more.
But yes, MFG's build things pretty stout. But it is impossible for them to predict and build to everyone and every circumstance of abuse. Run a weight box that is too small, but still allows near max lift, but almost no weight over the rears. This is the MOST that the front axle will ever see. But driving around is very dynamic. All that weight might not break the axle, but what about hitting a groundhog hole at 4mph? What about 6MPH?, What about full speed? ITs best to ballast properly and not take the risk. Things break no matter how well built or engineered. I would prefer to do whatever I can to lessen the stresses the machine sees. Proper ballast is one of the ways.