When I trained on 300's, we left it on all of the time. That's what the operator asked, so it'd be hard to explain not having done it, if the engine driven pump quit. Do they quit? Yup, just had one quit on an IO-360 powered fixed wing aircraft last week - aborted takeoff (well, it aborted itself!). Electric pump had not been on, because the check pilot had told me not to run it during takeoff. The runway was long enough... Now the pump is on all the time, its cheaper that the off runway forced approach I could have had to do!