You need a pressure tank mostly to minimize the pump on/off cycles. With the 30/50 switch properly working, the pressure will always be varying between those two values. If the pump (or auxiliary one) always ran when you were drawing water, then you might be able to keep the pressure constant. Some storage tanks are pressure tanks that rely on trapped air instead of a bladder, but the (newer?) versions use a bladder. The size of the pressure tank can determine how much water you can draw prior to noticing a pressure drop, but it will always be dropping with the pump off while drawing water from the system until the pump turns back on. My unprofessional opinion.