Why do you need to have gate valves fully open?
If a gate valve is partially open, the stem that holds and moves the gate will be in the water stream. If left in this condition for a long period of time, the water stream will wear away the stem, especially the end that attaches to the gate. The usual failure mode is that the valve is closed (stem pushes on the gate), but then as one tries to open it the stem pulls out of the gate and the valve will not reopen.