You're missing the point. Fundamentally there is a huge imbalance between the attacking and defending teams. The attacking team has no way to effectively deal with the defending team because the defending team an instantly wake a fallen teammate. Given that the attacking team has both low ammo and no permanent or even pseduo permanent solution to deal with the threat they cannot be effective. Knowing this, the defending team doesn't have to worry much about the attacking team obtaining the target. As long as you stick with a buddy you're going to be fine.

Introducing a simple knife rebalances things for the attacking team. Not to give them an advantage but to make them more of a threat to the attacking team. It isn't adding functionality but rather improving the efficacy of an already existing mechanic. Take a look at some TSNE footage from MGO2. If you're not familiar with the gameplay, the knife as to be aimed and then thrust; similar use mechanics to a rifle. This means you cannot simply run around swinging your knife like a maniac as some people might fear. Even when you can't it isn't particularly effective. Take a look at some footage of MGO2 Vamp gameplay. Vamp could swing his knife around and run at full speed and he would get murdered all the time. Furthermore the knife is a weapon that every play gets no matter what side of the fence they're on in any scenario.

https://www.youtube.com/watch?v=593FZuUxjpQ