Not everyone is a fan of group storyline quest, that's the main sales point of SW:TOR. RPG doesn't always equate to a party, otherwise games like TES never exist.

And it kills the immersion in that players who completed the storyline quests/missions can redo it in another group. What, did you just kill the big bad guy the 123456789 times according to the story?

It needs group mission, but at the same time it needs not to be all about group mission. A common ground to share, not only one or the other.