Maybe that's the thing. You see her as an addition.
Since I never played P4 on the PS2, I don't see her as an addition but an integral part of the game.
I like everything about her (slink, story arc, personality, even her dungeon!) and have trouble understanding the reasons given from ppl that don't.
But they usually are players that played the original P4.
So maybe that has something to do with it.
I think more often than not, the parts of a game that are integral to the experience can be objectively determined.
You can like Marie without labeling her in quite that way. I mean... Integral? Really?
You can finish the game having interacted with her no more than that first forced meeting at the train station. Integral, that does not make.