Swords were still used during WWII in some parts. And they were effective, until they came up against the fully automatics, which ruined them.
And guns have only gotten better.
Personally when playing I look at the MP as being like the MetalStorm, which would really mess you up heavily compared to a sword, sure, you can stab a guy, slash teh crap out of him, whatever. But is it really more effective than putting the gun up close to them, and just letting loose with a few rounds at high speed. The velocity of the bullets would only be higher due to shorter distance.
Heavier weapons, stuff like the bolter rifle, I'd be holding waist height, someone comes around the corner, it's pointing at their gut. I'd just fire off a round (RF) and it would go through them, faster than a sword.
Problem imo is that people are comparing a sword in it's scabbard to a holstered gun. These people are in a LZ, they have their guns out, and expecting enemies. From readied, pulling the trigger is faster than swinging a sword or thrusting a knife.
Knives have high killing power, kerblades even more so, I say raise the damage (Ok, maybe not, it's pretty freakin high) but have aliens RF, guy comes running at you with a knife, you'd fire off a few shots his way, if he's alive and able to fight, you're going down.
All the aliens have throats which they use for breathing, armour does not seem to protect that, if headshots cause higher damage, knives should own, once you manage to hit.