The idea would be that different map services are loaded and certain layers make up say a Utility Map "theme" but you could switch to another group of layers making up a Zoning Map "theme". The idea being that some layers from the other group layers could still be turn on individually to add to each map theme if needed. In a way...like the ArcIMS Layers on/off app http://help.arcgis.com/en/webapi/flex/samples/01nq/01nq0000005q000000.htm