The varistream takes over the job of the pressure switch, stopping the pump once the pressure has built up. If the pressure switch is wired in the there is more to go wrong, ie if the switch sticks for some reason in the off position, your pump won't work.
The reason why some wire them with the pressure switch in, is that some controllers use the pressure switch to stop the pump when pressure is built up and all the controller does is throttle back the pump. I've completely removed the pressure switches of mine and they work well no probs.
Simon.