Now that the obvious joke is out of the way, let's rap serious for a moment about the place of knives, swords, chainsaws and other sharp objects in online games.
It's easy to blame Call of Duty for popularizing knives as an instant kill in any close combat situation. But I'm sure that, in the beginning at least, their intentions were good. Instead of having people strafe around each other emptying their clips like morons, it provides a quick, decisive means to end any close encounter. It's also a good option to stealthily take out your enemy. In theory, it's a pretty novel idea. In practice, not so much.
The problems with ANY 1 hit kill weapons (knives, sniper rifles, railguns, etc) become immediately apparent when you introduce lag into the equation. It's infuriating to shoot first, only to be insta-killed by some asshole who is hosting the match. This happens with any kind of weapons, but when all it takes is 1 hit, there's much less margin for error.
Many games in the past managed just fine having melee weapons that took multiple hits, and were slow enough that someone shooting at you could actually take you out, even with an inferior connection. Knives do have their place. It's their implementation that these developers seem to be royally screwing up.
Gears of War and Call of Duty are the two worst offenders here. I've had matches in both where I did much better just running around being a dick and exploiting the horribly broken melee kills. Gears at least tried to balance it by adding chainsaw duels, even if they failed horribly. Modern Warfare 2 decided to void their bowels and just shit the bed completely, with ways to increase the speed AND range of your knife.
Some games like Uncharted 2 actually have a decent melee system, where it's not entirely connection dependent. But those seem to be much fewer and far between. For now I guess it's something we'll put up with until it dies out like hypercolor shirts, those Wazzzup commercials, and every other retarded fad.