It would be nice to have the option to drop the lines between polygons that have the same values for a specified field. This functionality used to be available in ArcPlot as the DROPLINE command and was widely used as a way to avoid creating a new dataset with the dissolve command.
There is currently a workaround available in ArcMap that is about 18 steps and only works in certain situations. I'm sure the programming wizards at ESRI could provide a simple 2 step solution for this problem. Just add an option to Features on the Symbology tab that allows you to select a field from a dropdown list. ArcMap could then dissolve on-the-fly the lines between the polygons with matching attribute values.