I actually just signed up for this website just to post this.
There is no realistic scenario in which a knife would ever be used as an offensive weapon by a trained operative when firearms are at hand. If one were to run out of ammo or lose all 3 firearms (some soldiers have a second backup pistol) the only logical choices would be getting the hell out of there (it's your life) or asking a friendly for ammo. Only an idiot would try to kill an armed man with a knife, as most likely the person with the knife will get his ass killed.
Another silly thing is how you have an instant kill melee while your victim is facing you. Is the guy you're stabbing retarded? Or maybe made out of straw? Maybe he's too preoccupied looking at the pretty flowers and clouds. If someone is coming at you with a 6 inch blade, your arms come up. He's not just sticking it in you unless you really don't care and want to die. Ever seen what a knife assault victim looks like? Their arms are sliced up to hell, but they're still alive. We haven't even factored in combat armor, helmets and the like. How could anyone with a brain expect a steel blade to pierce body armor and fatally wound the target so severely that he drops on the spot without even twitching or firing a round? The only possible way you could kill someone instantly with a knife would be to sever the spinal cord at the base of the skull / neck. Placing a knife there requires the person to be entirely subdued or entirely unaware. How often does that happen in combat?
Here's the icing on the cake. Games like CoD, Crysis 2, MoH and others allow you to instantly melee while holding any weapon for a 1 hit kill. So now 100% of the skill has been taken out of the engagement. Here's a very very common scenario. You're sprinting / running through a hallway or corridor and come to a corner. At the same time, an enemy player is also rounding that corner. Whoever hits the melee button first (often conveniently placed on a thumbstick click) insta-kills the other and walks away. There is nothing here that proves that the victor out skilled or out played his opponent. Some of the most difficult firefights to win are those that happen within 15 feet of each other. Your opponent moves much more on the screen and can easily slip behind you, which can be awkward with the tunnel vision in first person games. Anyone that has played Rainbow Six will understand exactly what I'm talking about. Turning 70 degrees and dumping a few precise shots into your enemies vitals in a half second is much more impressive than missing him with a melee swing for the insta-kill.
Counter Strike did it right. Equip the knife, plain and simple. If you run at somebody with a knife, you've already committed to knifing as you no longer have a ranged weapon in your hands. If you are seen with a knife out, you will probably die. Even if you close the gap, you will have to slash the guy multiple times, and still very likely get killed. But if you sneak up on him and bury the blade 4 inches into his skull, you deserve the kill. This is how it should be.
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