My experience is that if you select vertex object, one object, polymesh for material, you get a single item in the Assembly Room that upon inspection in the Vertex Room has fully selectable polygon groups as well as material zones.  I find this quite useable, as long as it is not your intent to rig the object in Carrara.

If you use the "one object per group" you get just that.  A Poser figure, for instance, will import with one Carrara object for every body part/actor.