Centrifugal pumps are counter-intuitive. As pressure on a pump increases, the power required decreases. In other words, if you put a valve on the discharge of a pump and begin to restrict the output flow, the power required by the motor will decrease. Most people already have it in their head that choking a pump back will make the pump work harder and use more power. This is incorrect as the pumps work is easier, and the motor uses less power as the flow from the pump is restricted.
Most engineers falsely believe that the RPM of a pump must be reduced for the power required to also be reduced. This is simply not true when you are talking about pumps with centrifugal impellers. With these type pumps the excess backpressure (created by choking a pump with a valve) is a free by product of horsepower. As the pump pressure increases, the weight of the water being lifted is reduced and the power required decreases. With centrifugal impellers, restricting the flow rate with a valve reduces the power required proportionally. When pumping fairly cool water, these type pumps can be choked back to very small flow rates without any harm to the pump or motor. It is easy to tell if an engineer or pump installer truly understands what they are talking about if he or she understands this counter intuitive property of centrifugal pumps.
When all you are pumping is cool water at a constant pressure, it is hard to beat the counter intuitive or "magical" horsepower characteristic of a centrifugal pump at a fixed speed. A variable speed controller would only be adding expense and technical complications, while trying to trick it into doing something that the pump already does naturally, simply, and inexpensively. This inherent property of a centrifugal impeller may seem almost "magical", but is simply one of the only laws of nature that is truly Counter Intuitive.
There are many good uses for variable speed drive units, but pumping cool, clean water at a constant pressure is not one of them. Pumping other substances such as hot water or hydrocarbons may cause damage to the pump if flow is restricted too much. Pumping some materials such as blood may be sensitive to impeller RPM. Other products such as concrete or peanut butter may be sensitive to pressure. Moving these substances with centrifugal pumps or when using any positive displacement pump, a variable speed controller could be beneficial. When all you are pumping is fairly cool, fairly clean water the counter intuitive property of a centrifugal pump with the simplicity of a Cycle Stop Valve for control cannot be beaten.