Why not leave an electric fuel pump on all the time?
The electric fuel pump is used to provide an extra safeguard against fuel starvation during critical operations in siphon fed fuel systems where either there is an increased risk of this happening or where sufficient altitude does not exist to resolve a problem such has switching fuel tanks or during takeoff and landing operations.  While one could in theory continue to operate the electric fuel pump continuously throughout flight operations, it it not necessary and just adds additional wear and tear on the unit.