Why not to use a Superknife
(I do not know how old this is, but I just noticed it), Nexon has a section on their "Player Profile" page that lists how many times you have fired every gun. If you look at one of a superknifer, you can see that they have around 200000-300000 shots fired with a couple melee guns, and around 50000 with normal guns. It is painfully obvious who is using modded melee weapons.
This is just another reason not to use them.