Someone who doesn't care about racking up the kills or points... And besides, as I say, it's undependable. You can't create a heavy trooper class layout if such a layout is a sitting duck for snipers because you never know if there's going to be a significant enemy sniper force or not. The game requires different people to be doing different jobs. You need to know there is someone working on keeping the sniper force down, and they need to know people are going for the objective, and are protecting our snipers, etc. The game needs more proper roles, something these open online games just can't get outside of organised clans.
Seems to me that trying to balance for that is nigh impossible. The reason most players run around with LMGs is because that's the gun for the individual who's just there to shoot things, which is 95% of players. Change the LMG to prevent that and they'll all go with the next gun, the AR. If the idea is to get more people trying more weapons, the solution doesn't lie in adjusting them, but making sure the game has specific roles, with the right tool for the job.