I would have to say then, with a hidden cooldown, this trinket blows for tanking/grinding. It simply won't fire enough to out damage a trinket with a "use" feature that increases spell damage for a set amount of time (I wouldn't think, but no way I'm racking my brain on that math =P). The spell damage increase is nice and high, but 1 mob every 15 seconds (optimal) in an AOE setting = fail, particularly as Shadow damage.
I change my statement: if I see this drop, and no Lock/S.Priest to take it, it's gonna get sharded.