Heavy means it's going to have a chance at causing some kind of damage, unlike the combat knife which can't penetrate Astartes Power armor. Heavy objects don't need to pierce armor, they smash the bone beneath it. In the Warhammer novels, the authors depict devastators using their weapons to smash enemies when it becomes a desperate brawl. I have never read once that the devastators threw down their heavy weapons and started stabbing people with their knives. What's with this "presuming" stuff? Just look at a devastator model, and you'll see that the belt feed or cables go from the front of the weapon to Space Marine's backpack. In other words, it's short so that it doesn't get tangled up in the devastator's legs. If they were longer, they'd risk getting tripped up by their own weapon. Astartes Power armor is almost immune to small arms fire, including their own blessed boltguns. The knife isn't going to be used to cut through armor. It's going to be used to target the joints while your opponent is just going to hack you in half with his chainsword. Furthermore, I don't read about Space Marines always having a knife. The knife is hardly ever depicted in the novels. I've gone entire novels without the knife even being mentioned. So, no, there isn't a good logic to assume everyone has a combat knife. Main Issue: Why would you put down the only thing that is going to give you a chance to kill your attacker? Kicking and bashing your enemy gives you a chance to use your heavy weapon and kill him. Pulling out the combat knife against a melee specialist is just fighting on HIS terms, not yours. You're already dead once you toss your heavy weapon away. Using the knife is going to get you killed. It is a terrible idea.