The reason I use LMGs is they help the team. Over the years I've noticed that when I run ARs, my KDR is higher, but my W/L is lower than when I use LMGs. I did some research and the main reason that militaries utilize them in the basic fire team formation is the volume of suppresive fire they can lay down. Suppresive fire is invaluable to a successful attack, whether flanking or direct as it softens up the defense for everyone to move forward. I agree with you, they are support weapons, but gunners using LMGs in the military aren't stationary. That is a job for mid and heavy duty machine guns, the big .50 cal. weapons.
A good gunner in COD is going to be someone who is really good at anticipating the flow of the map. This nullifies any speed disadvantages. I'm going to make sure I am running with someone carrying an AR and support them. While they are reloading, I can still have my weapon at the ready. If they need to move up, I can keep people down in cover. Being a couple of steps slower than the guy toting the AR actually helps this process. It naturally puts me in the position I need to be in. While the AR is scanning the ground, I'm scanning windows for snipers. The increased penetration makes the LMG perfect for this. There have been plenty of times that the guy running an AR in front of me doesn't even realize I just took out a sniper that had them in their sights.
I'm pretty comfortable with picking up weapons off the ground and using those where appropriate. Generally, I switch them out until I find an SMG. I do this as soon as I get my first kill, or sooner, if possible. If I am providing true overwatch with an M27, then I have a sidearm in my class. I'm good with pistols as well and the P226 is a great CQB weapon.
The only caveat to this is the ChainSaw, which has the same mobility as an AR. I use this LMG when I want to flank the enemy with a teammate carrying an SMG.
In essence, when we set up our classes, we aren't trying to play exactly like someone toting an AR. This is why you don't see many gunners and even fewer good ones. It demands a very specific playstyle and mind set.
I like the question. Militaries have been debating this topic since the dawn of the Automatic Rifle Squad, or Section, in WW2. The Automatic Rifle part originally, and still does, refer to the LMG. A soldier toting an M4 and such are still termed, Rifleman. The Colt in MW3 was actually in a competition to find an AR that could replace the LMG. It's a very interesting topic in both COD and real warfare.
Peace