Part of the reason it helps to throw out different classes is the fact that every player, at a high level, is predictable. If they're using the PDW, they're using laser. Doing a class that breaks the norms can unsettle people and mess them up
Even so, there's a reason why some attachments are heavily used more than others. --- The greatest victory isn't by points or kill/death ratios. It's causing your opponent to rage-quit.