It's situational. If you you have a tank who keeps aggro well you won't need it much, if at all. It's nice to have though for times when you have questionable or lower geared tanks or when fighting mobs that have threat wipes or can disable your tank. If the tank gets feared/gouged/MC'd/whatever and the boss comes after you it's nice to have the ability to drop you aggro.
Also, you don't have to wait until you actually pull aggro before using it. If you see yourself catching up to the tank in threat you can hit soulshatter and keep on DPSing without worrying about pulling aggro.
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75 Rabbit/75 Sheep/75 Coeurl/75 Eft/75 Raptor/75 Hippogryph/75 Puk
75 Scorpion/75 Wamoura/75 Pixie/75 Peiste/64 Sabotender
51 Bird/41 Mandragora/40 Bee/37 Crawler/37 Bat
Items no one cares about: O
Missions no one cares about: O
Crafts no one cares about: O