The KGC-0000 is a rare mech that's far more useful in the AI's hands than the player's. As part of the player's lance, the King Crab suffers from its low ammunition reserves. It has enough for 5 combined salvos from its AC/20s, and while that's incredibly dangerous to any individual mech it's up against, that's not enough to last it through a fight, especially when playing against the AI with its lategame tendency to use 2 or 3 assault lances at a time. As part of an AI force, however, it's only one of many mechs the player will be facing, meaning its lack of longevity isn't an issue because a single AI mech usually doesn't last long anyway, and all it needs to do is soften up the player's mechs enough for one of its many friends to finish off. The King Crab is one of the reasons Sensor Lock has a use even in the lategame so you can do as much damage to it as possible before it can bring those big guns to bear.
There are lots of great King Crab variants, but my favorite might be the KGC-005r from 3071. The AC/20s are upgraded to LB-X models and it now carries a total of 6 tons of ammunition, enough for 15 shots for each gun, making it dangerous for an entire fight and enabling you to make good use of both kinds of ammo. Its short range firepower is enhanced further -- because clearly it needed that -- with a Streak SRM-4. Its long range firepower of a Heavy PPC means it has less total damage than some other variants, but it has great damage concentration. All told, it can headcap at any range, and can singlehandedly punch holes in the enemy and then critseek them to death, making it extremely deadly. It's also highly durable, mounting a standard engine, 18 tons of armor, and CASE. And yet there's still more, because it also carries a C3 slave unit, so while it's up close doing King Crab things, it can also be enhancing the accuracy of its lancemates. It's a solid part of any force in its era.