What's Nick's value in Gatsby's party? (chapter 3)

In The Great Gatsby, Nick's role in Gatsby's party is three-fold: his an observer-narrator, a poet, and a moral voice who sets himself up to be better than the others.  This last one is most important, as the reader will align himself with Nick and Gatsby against the others, even though they are all complicit in the immorality of the age.

As observer-narrator, Nick reveals the carnival atmosphere of the party:

But as I walked down the steps I saw that the evening was not quite over. Fifty feet from the door a dozen headlights illuminated a bizarre and tumultuous scene. In the ditch beside the road, right side up, but violently shorn of one wheel, rested a new coupe which had left Gatsby’s drive not two minutes before. The sharp jut of a wall accounted for the detachment of the wheel, which was now getting considerable attention from half a dozen curious chauffeurs. However, as they had left their cars blocking the road, a harsh, discordant din from those in the rear had been audible for some time, and added to the already violent confusion of the scene.

As a poet, he assembles a collage of imagery that rivals T. S. Eliot's "Prufrock":

There was dancing now on the canvas in the garden; old men pushing young girls backward in eternal graceless circles, superior couples holding each other tortuously, fashionably, and keeping in the corners—and a great number of single girls dancing individualistically or relieving the orchestra for a moment of the burden of the banjo or the traps. By midnight the hilarity had increased. A celebrated tenor had sung in Italian, and a notorious contralto had sung in jazz, and between the numbers people were doing “stunts.” all over the garden, while happy, vacuous bursts of laughter rose toward the summer sky. A pair of stage twins, who turned out to be the girls in yellow, did a baby act in costume, and champagne was served in glasses bigger than finger-bowls. The moon had risen higher, and floating in the Sound was a triangle of silver scales, trembling a little to the stiff, tinny drip of the banjoes on the lawn.

As the moral voice of the novel, Nick attempts to set himself up as morally superior to the others at the party:

Every one suspects himself of at least one of the cardinal virtues, and this is mine: I am one of the few honest people that I have ever known.