One thing I've noticed as a Pistol\Sniper only guy is that the weapons later on in the game have such high accuracy that they are actually useful even when untrained. They don't do as much damage as the weapons I've trained with, but early Assault Rifles that have an accuracy of 1 or 0 are completely worthless untrained. The later ones (50-60 accuracy) seem to hit their mark pretty well at a reasonable distance.
Nothing beats the ridiculous power of my pistol and sniper rifle with two rail extensions VII mods each though. Its so funny killing 5 geth with 12-15 shots from a pistol.