The light machine gun isn't useful at range due to the spread, but up close it will shred through an almost arbitrary number of enemies given its high damage output, huge magazine, fast reload, and multiple ammo types. There are few weapons that give the finger to Cazadors quite as much as the LMG loaded with hollow point rounds.
Granted it is not a great weapon when you meet things with a DT higher than 15, but such enemies are few and far between. The biggest real problem that stops it from being the end of all things is the relative scarcity of 5.56mm armor piercing ammo. If you're packing the minigun, it seems like almost everybody has a couple hundred rounds of 5mm AP that they'll sell you, but not so lucky with the LMG. Like most automatics the LMG loses effect versus quickly against any kind of armor if you're not using the AP rounds.