To put it into BG terms, It would be like if Imoen, also a baalspawn, also born and raised in Candlekeep, also Gorion´s ward could be the heroine of Baldur´s gate and could turn into the Slayer too.
Many people dislike that kind of mindset because they do not see the party as a party, but cohorts of the main character. An origin character has the same importance for the plot as your character (heck, sometimes "is" your character). It´s not "game over" when your MC dies.
Good analogy, let me extend it some. Each of the DOS2 origin characters have a series of quests linked to their story -- one wants to kill her former slave master, another to stop a queen before she can commit an act of genocide . . . This isn't so different from BG2 where companions had unique quests but what
is different is that you, the custom player, don't get any unique quests. In BG2 each class got it's own quests in form of a stronghold. So it would be as if Imoen got the planar sphere, Minsc the ranger's cabin, Jaheria the Druid's grove and you were there just holding their extra equipment.
