The Nightmare Court's Objective (Spoilerish)
Thats not there main objective, thats just the current obstacle blocking them from it.
They need a tree to mass produce sylvari that are part of the nightmare from the start.
Like all evil organizations, there end goal is world domination or maybe world destruction.
Pretty much what XelNigma said. The corruption of the Pale Tree is just a step in flooding the world with Sylvari and taking over. Having another, easy to corrupt tree would make that so much easier.