For example, why does this Darth Malgus character show up all over the place? How many more people would play this game if it was Darth Vader?.
Are you kidding? He was in every single cinematic leading up to the game's release.
He kills Satele Shan's Master, Kao Den Darach. He kills Darth Vindican, his own Master. He leads the assault on the Jedi Temple and destroys it. He kills Master Ven Zallow, the leader of the Temple. He's there in the assault of Alderaan. He nearly kills Satele Shan AND Jace Malcolm by himself!
He orchestrates everything the Empire does. Every single flashpoint is a single stroke of his master plan. He has the players go to Hammer Station in order to obtain the Advocent technology. He has the players go attack Clan Varad in order to gain an allegiance of a competing Mandalorian Clan. He has players track down and kill Darth Revan so he can control the Foundry. He has the players perform in the Colicoid War games to have access to their weaponry. He has the players assault the Red Reaper and kill his Master's master, Darth Ikoral, to ensure that the Pureblood Hegemony will not return to the Empire.
Then when all of this is complete, he performs a coup d'etat, using every single element of all the previous flashpoints as his own tools to become the New Emperor so he could make the Sith Empire more like the Republic.