Currently we can use definition queries to form subsets and save them as layers which can be brought into a TOC and grouped as we please.
The downside of that, besides managing separate layer files, is that the records subset is static for any layer file, and if attributes change the records membership in the layer does not reflect that.
Why not make it possible in the symbology tab, under 'Multiple Attributes' to enter query expressions, such as 'NUM_HOUSEHOLDS' > 'AREA_ACRES' or 'ZONING_DISTRICT' <> 'COMMERCIAL' AND 'ELEV' > 500 ?
Thus we could enjoy the combined benefits of queries, categories and quantities all in one dynamic symbology.