Does it become more useful after I start unlocking STC modules and other upgrades (I think some of those fit in Heavy slots, so that could come into play then)? The boosted power/damage from its special ability seems to imply it should be used as a heavy intercepter, but that seems a little insane until I get better weapons (and its lack of speed works against it there, too) and by then, I'll probably have better aircraft. I don't even see a way to maybe "improve" it to the point of where I think it would be worth having, although maybe increasing the armor from 6 (the lowest weapon power of enemy craft looks like 15, I'm guessing they do the same 50 to 150% possible damage roll? Or do they work differently?).
Based on my experiences with my Red Codex play, Scarab was of very limited use. It does great damage with the weapons it can carry, but its terrible speed (1900) limits it to intercepting only things slower than it is, none of which are actually armed in any real way. On rare occasions you might manage to bait an HK into you, since the only way you're getting into a fight with anything that can actually shoot at you is if you convince them to chase you, seeing as the speed on the Scarab is basically nonexistent. Unfortunately, none of the big targets will actually ever pursue you. No HK battleships or anything. While it has potentially amazing firepower, the problem is that none of it matters if you can't get to the battle. The only thing that can salvage the Scarab from the junkpile of near-uselessness would be somehow finding enough orbs to make an Ethyr-X, wihch would boost its speed to 5500, enough to actually intercept things. Anything less than the top-tier Ethyr drive would not be sufficient to actually catch anything.
As there is nothing close to a viable source of Orbs that I know of, good luck with that one. If you somehow manage to cobble one together, THEN maybe you can buy a Scarab. Otherwise it's a waste of money. The very niche purpose of baiting HKs, none of which are capital-grade threats anyway, does not really justify the cost and hangar bay.
The same problem plagues the "ultimate" fighter, the Dragon, which is useless junk for all the same reasons: It's too slow to actually fight anything that would need that level of firepower. The result is that for all your research, you get a heavily armed transport without aux capacity. Totally useless.
Remember that red is not the only codex that exists. Try playing green without Scarab.
Green doesn't change anything that makes the Scarab not useless, far as I know.