I think there's a very fundamental flaw with comparing any of FFXI's expansions to Advent Children. Since I've played Rise of the Zilart (and that content was available to you from launch unless you imported before the NA release), it's become very apparent to me that the story from the original Japanese release was incomplete and intended for expansion. I agree with Del Money. Rise of the Zilart and Chains of Promathia are so closely related to the events leading up to the "defeat" of the Shadow Lord, it's hard to say they're not necessary to complete the storyline.

I'll give you the add-ons, ToAU, and WotG, though. ToAU and WotG were written by a different guy than the previous expansions, and although the original writer came back for the add-ons, there's little attempt to really join them to the main narrative (although A Crystalline Prophesy does tie up some very minor loose ends).