Marid Hides, Manzasiri Hair, Halgai Mane... the list goes on of monster drops that are required in substantial amounts but are a rare drop. For example, a set of level 70 Gathering gear (left side, no tools) requires 15 Gazelle Hides, 64 Halgai Manes, 73 Manzasiri Hair & 60 Twincoons. If you had to gather those purely from mobs it could take hours.