>>717554392
I think the explanation was fine.
In universe, your normal soldier doesn't have companions with warp and overload, so having infinite ammo against enemies with shields and barriers just wasn't working, when you had to wait for the gun to cool back down.
>The geth found that in an age of kinetic barriers, most firefights were won by the side who could put the most rounds down-range the fastest.
Having said that, I do think they would've kept some of the guns around for cases where you would run out of ammo. That's just inexcusable.