Actually, we have both types of engines (V2500 on the 321, the CFM on 319/320) and use the same procedure on both.
The only reason is to prevent engine wear. As our technical pilot states: The most important FLX-degrees are the first ones... Meaning that the difference between FLX56 and FLX58 is almost nothing compared to the difference between FLX39 and FLX41.
For pax-comfort we takeoff with packs on, only if the FLX becomes below the said 45 we switch them off. And even then some captains don't like to do that, and leave them on for pax comfort, if we don't need them for performance.
P77