As far as strain on front end parts goes, I can't imagine any difference between using wheel weights and loaded tires. The weight is still carried by the wheel/ tire which is in contact with the ground (hopefully) and not carried by the chassis. During turning I'd think a FEL, or suitcase weights must strain the front end a lot more than wheel weights, or loaded tires.
The only thing I can guess is that by adding liquid you reduce the amount of compressible (air) inside the tire. Maybe it strains the tire/ rim more???
Front wheel weights would actually reinforce the wheel, so thats a plus for their use.
My current ride, a NH TC40D has loaded fronts. I bought it that way, and wouldn't have even known except for removing the wheel during maintenance. I imagine they were filled by the NH dealer when the tractor was new.
I don't know. Im sure Kubota put it in the book for a reason. I'd just like to know the reason.