User blog comment:InfinitysCross/I had a question.../@comment-1942617-20110802191826/@comment-1388547-20110802195705

It's annoying to run out of ammo, but its main effect is as a balancing tool for the player's weapons. Most of the sniper rifles, and some of the heavy pistols and assault rifles, would be overpowered if they weren't ammo-limited. If I could spam shots from the Widow/Carnifex/Mattock with impunity, the game would be too easy even on Insanity. ;-) Since it's also possible to kill things with powers in ME2 (which was very difficult in ME1), and powers never run out of ammo, it's not too crippling an issue for me.