There is literally nothing wrong with light pen as a concept.
The game encourages build diversity with armor gating because otherwise you would just run BIS light pen weapons for everything.
Same reasoning with heavy armor and tank armor, some form of anti-tank is required to make players consider the team's loadout and think about what to bring.
That's all not to say that light pen weapons don't have a place, every faction, every mission, even on D10 have chaff enemies that can be killed with light pen. Their whole purpose is to have plenty of ammo so it can clear chaff quickly and effectively, nothing spectacular. You start making it so that more enemies can be taken down with light pen and all of a sudden medium pen weapons have no reason to exist.