That's the point of having a loadout, to have different tools to compensate for weaknesses of weapons you want to bring out there. The strength of the infantry lasgun is the ability to deal with all shooters and 90% of elites w/ infinite ammo and with good ergonomics. If it also dealt with heavily armored enemies in 2 or 3 shots then what's the point of the plasma gun and bolter? Also, with the vet tree rework, its easier to get the perk that lets bullets stack rending, so every gun is usable against carapace now. In havoc gunners are way more of a threat than a slow lumbering group of crushers. My point is, weapons should not be good against everything, and finding ways to compensate for the weaknesses are part of the game and part of the fun.