I'm the guy with two complete LnL powder measure systems.
I bought the second one to get the "Bench Rest" rotor which is unfortunately not sold ala-carte like the rifle and pistol rotors. The bench rest rotor is sized half way between the rifle and pistol rotors. It is sized better and throws more accurate charges for small rifle cartridges like the 223.
If I ever start loading larger rifle cartridges that need more powder then the bench rest rotor can dispense (30-06 and above) then I'll probably buy a third whole powder measure just for the rifle rotor instead of swapping the rotors around.
It is a matter of convenience, not necessity. It's just a lot easier to swap the whole powder system at the LnL bushing then it is to swap powder from the hopper, disassemble rotor, install new rotor, and then swap hopper on to powder die.