I actually kind of disagree, melee is never 100%, I've missed, and been missed by people PLENTY of times. Ever been in a window capping a few guys further down the map, and hear a whisper of sound behind you, whip around to find some idiot trying to knife you?
That's called a failed Melee.
I think melee is extremely important, honestly. Sometimes, reloading is the difference between life and death, sometimes you simply don't have the time for the clip. So, you smash them with the butt of your rifle. Stab them with a bayonet, or knife them. It's kind of strange to think that any combat soldier, rebel, or veteran of any kind would go into battle without some kind of backup weapon (and I don't mean a side arm). Knives are not only vital if you run out of ammo, but for utility purposes also.
A knife to the spine, or heart, is no different than a bullet to the head or chest. If it doesn't kill you, it's going to seriously take you out of your gameplan.
I think your issue, is really more of a problem with FPS' in general. A lack of serious bullet/impact physics. Ever notice how there's really very little difference in a 5.56 round versus a 7.62 in games? I would personally LOVE to see them implement actual physics to bullets. But, then you run into the 'realism' problem, where everyone bitches so much because...oh my god, I can't shoot 4 miles across the screen, and so on. Realistically, they should have to arc shots beyond what, a third of a mile? Even less?
Fix those issues, and you'd probably get more of what you're looking for. As for one-shot kills with knives? Well, if it's a 5+" blade, it's got plenty of length to pierce your heart.