Bioware. This is the key reason for adding companion characters to Star Wars: The Old Republic. A lot of MMOs like to build the adventure around one lone character conquering the big bad. In a world with a lot of people who are all the best, why would anyone need anyone else? Bioware's Creative Director and Lead Designer, James Ohlen, tells IGN in an interview today, "Companion characters have a significant impact on the player's experience. They are integral to the player's storyline."
So other characters can play a significant role in storyline progression and plot. What about combat? Sure, a lot of games have characters you can talk to and interact with, but they aren't important to battling the enemy. "Each companion has a specific role, whether it be additional DPS, tanking, healing or crowd control. Companions also have special abilities that can be activated to significantly change what's going on during a battle," Ohlen says further in the article.
Read more of the article on IGN. Then let us know what role you think companions are going to play, and how you would like to see them used in SWTOR.
[Update] the official site has been updated to include companions. In KOTOR tradition they have a blue twi'lek: Vette.