So there's a few reasons for the Nightborn to join the Horde. They're not -good- reasons, but they're reasons.
1) Liadrin's Support and Tyrande's Distrust.
2) Cultural Alignment with the Blood Elves while the Night Elves are still fairly Anti-Arcane.
3) Alliance's focus on the Light, which the Nightborn don't share.
And there's a few reasons they shouldn't join the Horde.
1) Undead are culturally verboten in most societies
2) Trollwars, though I -think- that might've been more EK than Kalimdor.
But what reasons, specifically, do they have to join the Alliance? Beyond "The Horde Stinks" and "We've got Nelves" I'm mostly coming up blank.
In learning the history of the different wars between the two sides they'll learn that the (current) Horde and the Alliance have been aggressors and victims of each other as the tides of war run back and forth. Each side will tell their side of the story and present the other as monsters and themselves as victims and heroes (As we can see on the Forums, here, where Alliance and Horde players devalue their side's aggression and aggrandize their heroic traits).
And both sides helped during the revolution (Or, technically, it was 'faction neutral' class orders that the players represented). So that's really not much of a for or against on either side.
I'm open to suggestions.