first of all, knives are for stabbing. not slashing. that being said ... what do you think happens when you insert a 7 inch knife below the center of the left shoulder blade between the 3rd and 4th ribs pointing upward at a 30 degree angle toward the center mass of the chest? ill give you a hint. it perforates the left lung and enters the heart. granted you may not die for a minute or two but you can't yell for help either.
in RL ... (real life) the knife requires the most skill of any weapon that a soldier has at their disposal due in part that when used properly it allows them to incapacitate an individual in such a way that it makes ZERO noise rendering the ememy "combat ineffective" without making their presence known. why do you think they teach knife tactics to operators in the special forces community? because their ability to go unnoticed is absolutely critical.
wow. what a jerk.
First off, knives are for both slashing and stabbing, that's why they have the sharp edges and the sharp point, although I suppose it comes down to what kind of knife we're talking about.
Second, since there are no "canned animations" you're not going to have anybody carefully placing their knife between the 3rd and 4th rib at a 30 degree angle toward center mass, so it doesn't provide support to the idea of OHK knife attacks in Brink.
Treat the knife as a weapon that will be used in a "knife fight" situation and not a "tactical operation" situation. In that case it is much harder to kill in one shot, particularly when the opponent is aware of your attack.
And in RL the knife does not require the most skill, it requires a particular set of skills. Is it harder to use in general than a heavy machine gun? Yes, but that doesn't mean it takes more skill to use than the HMG is within more specified situations.
Moving on to the main issue though, when the game is a shooter and bullets are limited in strength in order to maintain a particular level of gunplay a knife that can one hit kill regularly becomes a severely OP weapon. It doesn't matter how much skill you feel it takes because if I shoot you 6 times and you don't go down only to stab me once and drop me, that is problematic.