When it is close to 100% humidity here in the north east, the air is just about saturated with water. So I don't see how any garment, or one with a base layer underneath, will assist the athlete in unloading sweat. Assuming that the close fitting jersey is made with similar polyesther hydrophobic materials as the base layer, why would two layers of these materials be better than one?

I have found one benefit to wearing a base layer with jersey under high humidiy and termperature conditions. For rides of about an hour, only the base layer gets wet, allowing me to use the still dry jersey another day. On the other hand I don't believe that there is any cooling advantage, that I can discern, with the added base layer.