Try an RPD with a grip and stopping power if you really wonder what LMGs are useful for in COD4 or MW2. Mobility and hip spread may not be impressive, but the sheer amount of pain you can hurl down a choke point without having to stop is downright amazing. It may as well be an automatic sniper rifle with a 100 round belt.
The catch is that LMGs aren't for running and hopping around as the SMGs and ARs are. If you're forced to constantly move then they'll be at a huge disadvantage. This is why LMGs haven't been very useful in the last couple of COD games- their maps and spawn systems really punish you for guarding a choke point from one side. The enemy will just spawn behind you eventually. And even if they don't they'll probably just flank you the old fashioned way or spam you with tubes and rockets.
LMGs really are great for the traditional defensive role, the only problem is that as far as MW2 and BO are concerned the traditional defensive role is so impractical to carry out thanks to the aforementioned reasons. So, what does this mean for MW3? Either A: LMGs will be re-balanced to make them more effective while mobile (essentially just making them into heavier ARs), or B: setting the maps and spawns so that defenders actually have something to guard.
