*Nod, nod* A Revan-ish villian is very interesting. Kreia-ish as well, because they are calculating and complex, not obsessive and blood-thirsty. It is much more terrifying to face an enemy who can plan your every move, spot your weaknesses a planet away, and destroy a powerful opponent without throwing everything they have at them like Malak does.
Plus, they are more interesting as characters when they aren't completely at the mercy of their bloodlust. Kreia or Sion/Revan or Malak? Its like comparing an assassin with a wrestler... most people would find an assassin much scarier and more dangerous if they actually faced them.