A knife would be very valuable and logical if properly implemented.
If a person runs out of ammo or ends up right behind an enemy while rounding a building what are they to do? Emote them to death?
If there were just some form of melee attack - any attack - even a shove or butt with a rifle to gain some distance it'd work wonders. Are we really to run away and scrounge for ammunition simply because Bohemia failed to put in such a practical solution to an apparent issue?
Given there are balancing issues to take into account, such as how many hits equate to a kill or how often an attack can be initiated (I mean sniper rifles can kill with shots to the head or upper torso, but they also have limited ammo and some have slower reload) There is a genuine need for such a feature and they really ought to consider having knives in this game.
From a tactical standpoint, for instance, what if you've got no silencer to take out a guard and you wish to remain stealth? From a logistical standpoint, a player happens upon an enemy just around the corner and there's a structure in the way, what do they do? There are limited options and there are points where one just wants to say forget the gun, I need a swift and reliable attack.