I understand the theory of shareware, and how in concept allowing temporary access to Black Ops 2 multiplayer could promote sales. In reality though this just does not happen. Undecided consumers take advantage of the free weekend which alone would be great for everyone. Unfortunately though, a much larger group of people take advantage of the free weekend. This group of people is only interested in "griefing" and "hacking".
What naturally happens is: your potential customers are given the worst possible experience Call of Duty has to offer. It is extremely unlikely a player on the fence about shelling over $60 is going to do so after 3 days of being exposed to racists, screaming jerks, hackers, and everything else that comes out of full anonymity with nothing to lose.
Not trying to say you guys only care about money (it would be my top priority if i were you though).... but free weekends are costing you a lot of money.