I believe they need to go, just because that they seem to have replaced firearms as the killing weapons. If I was to stab someone in the neck, let alone the chest or back, they would still be alive long enough to shoot me or even to make a full recovery in the hospital before coming back to the fight a couple months later. But if you shoot someone virtually anywhere, you're likely to kill them just from the shock and damage to the nerves or blood-loss, or the fact that their brains are on the wall behind them. Now knives have their uses in combat roles, but by modern standards, they are outdated. Honestly, the odds of actually ever employing a knife in ANY sort of combat situation would be your final resort after your puny pistol is out of ammo.
Edit: A alternative would be by making melee a 'Skill' and not a instant killing tool overused by everyone in games like COD. I'd like to see a game where characters can only sprint for VERY short distance, and normal movement speed is slowed down realistically. That way knifing people would be a challenge, you'd be required to actually sneak or outflank people with skill and stealth. Rather then the hundreds of people who can sprint for a unlimited time and crank their sensitivity so high that they can literally run up and knife you in the face.