It doesn't seem that necessary to go into detail about how the princesses can disagree when we already know that they can disagree.
It does when the thing they're disagreeing on is:
A: the reason for the whole plot
B: something the fans wouldn't normally expect them to disagree on
C: When the thing they're disagreeing about is more important to the plot than the disagreement itself
Remember, that a major plotpoint of the fic is "the Elements of Harmony are
not on the ponies' side. And this is the first hint we have of that.
And it was relevant to the Changeling character?
Not as much, but Luna's view of Chrysalis provides some perspective.
It also explains how The Schism could happen without Luna and Celesia being opposed.
EDIT:
Personally I didn't find much of it all that gripping. The only part I really wanted to be developed in more detail was the relationship between Celestia and Luna. There's a ton of potential conflict between them in whether they want to use the elements or not, and I'm way more interested in that conflict than in finding out how the elements/magic/whatever really work.
But ... this really is what it's about. Twilight comes up with her own opinion, in fact, where she disagrees with
both of them. We've learned practically nothing about how the Elements work yet except that they're weird and powerful. The differences in opinions on how the Elements should be used is what's caused all the problems that the odern characters are dealing with.
edited 19th Apr '13 3:15:52 PM by Sereg