IMO it depends on how loose your jersey is. If your jersey is skin-tight, and for hot weather it should be, then adding a baselayer simply increases insulation. If your jersey is loose, so that it's not saturated with your sweat, a baselayer might help if you unzip half-way so you can get air over it. I've tried it both ways. The biggest difference is of course when climbing when there's not as much airflow. The closer your sweat is to airflow, the faster it will evaporate. When I take off my tight climbing jersey, it's dry by the time I get it off.. I ride in a dry climate which can get quite hot.