You say don't make the drops useful, yet it seems most of the threads that pop up about open-world NMs with long spawn times usually centers around competition between various LS's to claim said NM. There's not much of a point of putting special spawn conditions on a mob for a vanity-only item. Hardcores (the definition here being people that will camp something hours at a time) are going pursue something that actually progresses their character, not some rabbit hat with no stats.