Have to have a pistol equipped to have a knife. That in itself is limiting.
Knives do MASSIVE damage compared to other melee attacks, but without the knockdown. Blaze away for a few seconds with your machine pistol as you charge, then knife and finish the job.
A skilled knife wielder irl could kill silently and efficiently with a single hit, but so what? A skilled sniper could headshot for an instant kill, but that's never going to happen in Brink either.
A skilled operative would only try this if it was completely necessary, its very dangerous and unpredictable to attack someone with a melee weapon. This would only really be used if it was a SF team that had equipped themselves for a day/loud mission but got themselves in a situation where stealth was key. It would take a while to do the job, slashing main arteries and possibly peircing a lung whiile also holding the persons mouth shut means they would not be able to fire their weapon as it would be slung to their back. It would be much better to have a marksman with a suppressor take a shot or incapacitate the enemy with a silenced sidearm.
This exerpt is taken from a SF soldier who wrote down some guidelines for many different things, tactic related, for induviduals or the team to follow. Here are his thoughts on silent takedowns:
Silent Sentry Take out
Everyone has seen a movie where the good guys sneak up and cut the throat of the bad guy with a knife. This is one of the more difficult methods. Silenced weapons are preferred, bows or crossbows are secondary, knives are used when nothing else is available (like in most infantry units). A garrote can be used also.
Whenever a sentry is designated for 'removal' usually a pair of troopers is assigned the job of removing one or more sentries. One man employs the weapon (usually a knife) and the other man remains ready with his assault rifle (or machine gun) in case things go wrong.
More than two people can be used in a sentry removal team but silent sentry removal will be detailed in another section.
So as you can see, the knife is a last resort weapon for anyone actually in the field. It was put into games to balance and speed up close quarters engagements. They are not even one shot kills IRL, As you can see the garrote and the knife are interchangeable.... this shows you how effective the knife really is. When most people think of knife kills they think of dramatizations, it is just as loud, messy, and time consuming as strangling someone to death. Hope this helps.