However let us not forget that we are combating against Aliens! These creatures seemingly wear far superior Armour than their human counterparts do, meaning that a Terran weapon that would bring down a normal human in a single shot may only wind an Alien, but the reason we bring these weapons with us is because it is the best we can offer. Also we are combating a viarity of Aliens in different environments and situation from close combat to long distance, each Alien having different Armour, tactics and weaknesses, bringing weapons that can be adapted to these situations is vital.
Simply put, normal Human conventions of warfare are not going to apply easily against the Aliens. It is like comparing Archers to WW1 Riflemen. The riflemen have a big advantage with their better weaponry and ability to use tactics the Archers would never be able to employ, however the Archers could still kill the riflemen, eventually.