Is TBN a bad skill?

[Absurdity]

I would say it's closer to bad in the sense of if the boss is too weak to pop it in the opener, like in E6s,

You could always take off your gear, put TBN on yourself or your co-tank (who would have to be naked in that case) and put it back on right before the pull. TNB scales with the HP your target has at the time of casting it so no gear = less HP = lower shield.