Disruptors are NOT heavy weapons. I don't know what the idiot at ADB was smoking the night he watched Elaan of Troyus (The only TOS episode to show Klingons firing weapons until the remastereds came out) but it is very clear that they were NEVER supposed to be heavy weapons. The encounter with the Klingon Ship went a lot like the old Soviet/USA Buzz jobs that fighter pilots would do in neutral/disputed areas (typically over ocean areas not claimed by either) a pilot would buzz another pilot in an attempt to provoke an incident, the side that fired first would have to face the wrath of the United Nations... condemnation from all the non-aligned nations... etc... When the Klingon Ship buzzed the Enterprise, it was an attempt to get the Enterprise to go to Battle Stations and make a move to defend itself. Which, because of the sabotage, would have destroyed the Enterprise. The destruction of the Enterprise at that point could be laid at the feet of sloppy maintenance, under-supply, a drunk engineer, a whole slew of things, Klingon Sabotage would have been quite a ways down the list. When that failed, and it was clear that the sabotage was detected, "Captain Surrender" moved in to do some damage control, but because of the Organian Treaty couldn't outright destroy the Enterprise. The destruction of the Enterprise at that point would have been solely to blame on "Captain Surrender" and the Klingon Empire would have suffered the consequences of such action. Therefore his goal was to disable the Enterprise further, so he could board it, and dispose of the evidence of sabotage. You do not fire heavy weapons at a ship you are trying to disable, you might accidentally destroy it, especially since the sabotage was to the Enterprise's main power systems, with her weakened shields, a single heavy weapon would have probably destroyed her.
Why there needs to be limited Heavies: Supply. D2s campaigns without supply rules degenerated into checkerboard matches with people grabbing whatever hexes they needed to grab, the only use for bases was to repair at, and if you flipped a planet hex, that suddenly became an instant repair/reload source. You could make a hard code rule that a ship that is OOS would suffer some arbitrary 15-20% reduction in combat efficiency, but I don't like that because a ship just cut off from supply (or a player logging in for the evening after work finding him/her self suddenly behind lines) is suddenly much easier to kill. It also limits the ability of deepstriking, someone leaving a border base, as soon as they cross into enemy space and slip through the perimeter, they suddenly lose combat ability? Limited Heavies makes it so you can still have supply rules without inflicting arbitrary penalties for being OOS. Once you've used all your Heavies, you've reduced your own combat ability. If you're careful with your heavies, you could go on for weeks without a resupply.
Now... I answered my own question as to why you can't replicate Torpedo Casings earlier this morning. If a replicator was capable of creating a Torpedo Casing, all it takes is a warp powered ship, and every Tom, Klank, and Harry would have the ability to make as many of the 2nd (3rd once the Quantum Torpedo was invented) most power conventional weapon in the known galaxy as they had fuel for. No way would anyone want that, not even a Ferengi. So they make it out of something that can't be replicated, and you have to stop off at your local certified arms dealers to reload your Photons, a small price to pay to not have everyone running around with a thousand of them.