Been out here 40+ years and had a tractor all that time.   I thought about a ballast box a few times.   For me - a ballast box is of no use.   I have VERY FEW really tight places and have no business with my tractor in those that are tight.
I have 1550# of RimGuard in the rear tires and a 1050# rear blade on the 3-point.
The 
M6040 Kubota is just not a tractor for tight places.
However - I do see the definite advantage of a b-box if you have a smaller tractor and need to do work in tight spots.   I see some very "enlightened" designs for b-boxes - increases their usefulness.
I think of a ballast box as most other 3-point implements.   If you need it - fine.   Otherwise - wise money says use a 3-point implement and get more bang for your buck.         JMHO