To me, it's not a question of "justify". It's a question of overall budget and the goals of my vacation.
If I'm going to do a ton of "extra" stuff" like ride photos, park tours, souvie shopping, boat or other equipment rental, recreational activities (parasailing, fishing etc.) spa treatments and fine dining, then I might stay at a moderate (I'd never stay at a value, just my opinion) instead of a deluxe so I could afford those things. If I plan on doing only a few or no "extra things" then I'd stay at a deluxe, because I have more money for it. If I want to stay at a deluxe AND do a ton of "extra stuff" then I save up more money to do so. (notice how NONE of these options are shorten the trip!

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So, I look at it backwards, I guess. I have $X,XXX.xx to spend and X amount of vacation days. What can I get for the money? (and this goes for ALL vacations not just WDW). Then I book and plan until all the money is gone! If that means I can afford a delxue and still do everything I want, I stay deluxe. If I can't then I either cut down the amount of extra stuff OR I downgrade to a moderate OR I save more money and increase the budget so I can "have it all".
At the end of the day, it's YOUR vacation. If the $300 a night is within your budget, and you want to stay at the GF, then do so. If you can't afford it, or would rather spend the money doing a character meal or a dinner show, then do that instead.
At least, that's how I feel.