I asked Lead Flashpoints and Operations Designer Jesse Sky about the respawn timer on the Nightmare Pilgrim. He said that now that server populations are more dense on our destination servers, it makes sense to reduce the respawn timer. In a future update, it will be lowered to 30 minutes, and we'll continue monitoring feedback about it!
Fantastic news, this will definitely help on our server. There is a thread that was created on the Harbinger forums entitled Nightmare Pilgrim Etiquette so other guilds could attempt to 'schedule' Pilgrim raids around one another. This new/future timer sure beats trying to force a second or third instance of Voss by flooding the planet with players.