12-13 tons of armor hardly make it a glass cannon, it's just a specialized design that functions poorly as a solo unit without lance-mates to keep the targets engaged outside of a close range bracket. The ER-PPC isn't a bad idea - it does let the Mech continue with its long-range support role to a degree even after its ammo bins are empty. Just that while those (substantial) bins still have reloads, the PPC is inferior to missiles at long range, and would be inferior to a couple of Medium or Large lasers in its place inside the LRM's minimum range band.