I don’t think dust/chip control is the purpose of building a box like that, and I if you do want chip control there are easier ways to achieve it that don’t limit the accessibility of your workpieces, like temporary cardboard shields for example would be very cheap and easy and removable.
To me the purpose of a box is so that it is not attached to the table, and you can place the machine on a large workpiece or have a flexible table (or multiple tables) for different kinds of work. I built one for this purpose but if you are building it onto a table it is not an improvement. It’s only for if you want a portable device.
As for a box as an alternative to legs for stiffness, I don’t think it’s justified because the legs deflect much less than the center assembly. The downsides of material cost and accessibility of the workspace would far outweigh the tiny stiffness improvement which will barely be measurable.