In an interview with Nowgamer, Splash Damage's Richard Ham explains that while testing a multiplayer game with hundreds of players is important the developer feels that "with some betas being used purely as marketing tools, some players treat a beta the way they would a demo, and they expect everything to be fully working and polished."
This is something that burned Splash Damage when they were developing Enemy Territory: Quake Wars. Because of this, they are skipping having an open beta for their next game, Brink. "For Brink, we consciously decided to keep the beta private and put it in the hands of folks with prior beta testing experience," Ham said.