About a month ago Devil240Z posted a topic how he hates USAS-12 users. But since then the issue has got worse. It started with 1 or 2 players on a team using the weapon, but now it seems as if every other player has the shotgun setup with frag rounds.
In Bad Company 2 there was the NeoStead 2000 issue, where players could use it as a sniper rifle, but at least you needed to line up a shot for it to work. The difference with the USAS-12 is that you just have to aim in the general direction as keep tapping the fire button.
I feel that DICE need to balance the issue as the majority of players are using the exploit, making the game in certain situations completely unplayable.