Because you can put multiple categories on a workset the only way to halftone all items on a workset is to check each box in the visibility graphics dialog to be halftoned. It would simply be more efficient if you could halftone an entire workset.
One reason you might do this is for coordination. Say you have a lighting layout with a lot of overhead routing, you may want to leave all routing on for coordination but halftone it. You may need to halftone a large number of categories to get the right look, whereas if you could just haltone the workset you could achieve this much faster.
Another problem this would solve is the ability to halftone some of a category but not all, supose you had a dozen families all under the same category type but only wanted to halftone a few of them, you would need to create a filter first (probably using a workset!!) to achieve this. Why not just add the haltone check box to the workset?