For the Bearers, a mission can begin -- or end -- at any time. In this case, they're tired, hungry, completely out of bits, and stuck roaming the streets of an unfamiliar town until Sun-raising. So while they're waiting for their ride home, why not take advantage of that costume party? Just go in claiming to be masquerading as the Bearers, keep the circulation centered around the food trays... in Rarity's opinion, it's the best possible way to get through the night. And it's not as if they've ever had something go wrong at a high-society party before.
...except for all the times when it has.
The time to learn what everypony thinks of you is when nopony knows you're there...
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Dedicated to Harpo Marx.