I think that just came up not because of the damage it does, but how accurate/random people can be with it. It takes just a bit more skill to cook a grenade and throw it into someone's gut and kill them, than just pulling the trigger and impregnating them with a shell.
Plus on certain maps, people equip an M16 with the NT and an M4 with it as well, giving them 4 shells to lob at the spawn point/surrounding area, which in hardcore, blows if they get you.
It's just cheap, but all shooters will have them, and nothing will ever be perfectly balanced, because that's how it is in 'real life' i suppose. All guns will kill you, but some will kill you faster. i.e. .22 pistol vs .50cal rifle. oh well