Why can't we be there to give our opinion? It's people who hardly play PvP that will give you the best insight if theres something is wrong with the mode or not. Why? Because they are not bias. I'm an engineer and I automate processes, we'll have a team working closely on a process and to us it may be perfect. But when we let people who never seen it or use it before, they identify things that can be improved or wrong that we never thought about because we are not the end users.
By letting a new person play PvP, they can comment on the game, from there the developers can easily gather common themes that these new player have comment on. Simple things like, "oh, I notice I get snipe a lot", " why is that guy not taking damage", "I wish we have different guns" etc....
Beta testers are pretty much people who fall into this category. What do you think a beta release of games are for?