I think Bioware made a mistake with Sith being playable. It should have just been lightside/darkside and people fight along localized factions for control of the Senate, the Republic, and all its territories.
Sith are supposed to be evil, so evil I can't imagine why anyone who is a fan of the Star Wars movies and franchise would want to play them. They are supposed to be more evil than even anything we as humans here on Earth are. And Bioware does a great job at making them close to as evil as us.
I want to play with dark force powers and play bounty hunters, but being a world destroying tyrant? Not what I'm looking for, and I'd be fine with being less powerful than Sith npcs because my characters can't be as evil.
By the way I play a backstabbing, executing, stealing, lawbreaking, chaos-loving smuggler: evil enough to be a concern for the 'good' people of the Star Wars universe, but no Sith.
I know it's cliched but having Sith as a playable faction is like if there was a WWII MMO and you could be Nazi's. I don't know why anyone would want to do that, and that Sith outnumber Republic players on nearly every server is disheartening.