Also, Starsector combat is currently balanced around ships having to close with one another and a priority on maneuver as a tactical concern. Making very-long-range weapons too common and effectual would make maneuver irrelevant and turn combat into a slideshow of "Who Has More Missiles?" Small LRMs are especially vulnerable to becoming a spam-happy win button, because it's much easier to hit PD saturation/overwhelm shields with small missile mounts.
Pilums are weapons you deploy to harass your enemy (by occupying PD, scoring rare lucky hits, and generally cluttering the battlespace), not kill your enemy. At 7OP, they're not bad; you're rarely going to score hits, but 500 HE damage when you do is pretty impressive, and spraying a ship with ion pulser fire in the middle of an LRM cloud is a good way to see that ship evaporate.
That said, I think there are a lot of ways to do more compelling LRMs, which is why I tend to design LRMs that hit more often for less impressive damage. One of my favorites currently is a huge AoE fragmentation warhead with a small on-hit HE explosion, and better engine stats so they actually hit more often (and sometimes take out two or three fighters at a time). Even given the loose design constraints of balanced LRMs (medium or large mounts, and a sliding scale of low damage and high hit rate, or high damage and a low hit rate), there's a decently large possibility space.
So vanilla could probably use another LRM option, yeah; but I doubt making LRMs into reliable damage-dealers would improve the game.