They've been weapons for a
looong time, even the 3.2 list from the Fanatic archive (circa 2006ish) had them that way. Corey put some thoughts on why in the Raiders unit design notes:
Quote:
Scarabs were converted to a weapon system for units that can
“produce” them for two reasons. One, it was simply easier to
handle them, then to try to handle “Spawning”, come up with
costs, or play with a unit that you would have to have in
unmanageable numbers to be effective. Secondly, it reduces the
amount of models that would have to be made for the list, and I
personally would really hate to have to base figures the size of
the head of a pin…
Not the most compelling of reasons, but there you go.
Maybe just move the weapon stats into a Character unit, and give the Warrior formation an option of purchasing 3 of them for 25-50 points? That way you're not giving Warrior formations a bump against shooting or cheap extra stands for assault resolution, which making them their own expendable unit would do.