Probably the biggest defense for the Unique tag is for extremely exclusive gear, like Raid items. It's gotta sting to see a rare drop you need like the Genji Fending body get need-lotted by the other tank, who already has the piece, because he wants it to glamour one of his other tank jobs.
I honestly can't see the point of it on stuff like Tomestone gear, though. Seriously, SE should WANT players grinding out more Tomestones, since it's further incentive to keep playing the game even when you've met other goals.