The purpose of having a hot water circulator is to have hot water at the fixture as soon as you turn your faucet on. If the pump were to turn on on-demand (when you turn on your faucet) you would have to wait for the hot water to reach the fixture. This kind of defeats the purpose of the hot water circulator.
Hot water circulators actually use more energy and waste heat from your domestic hot water. I would recommend doing without this if you can.