Let's say Black Ops is released and everything is fine for the first few months.
But then lots of people start to use the same guns. In every game you walk into, that specific gun is bound to be there and used by 50% or more of the people there, much like what happened with the M16 in COD4, MP40 in WAW, and anything that rhymes with R in MW2.
Should Treyarch nerf these weapons once they get to a point like this?
No. If they nerf a certain gun everyone would go to another gun. Then they would nerf that one and everyone would go to another. The cycle would just go on and on.
That would make the game turn bad.
Also, people need to choice to choose any gun they please.