The tradeoff simply isn't worth it at this time for BCB, in my real tanking spec I would rather have Mark of Blood or Rune-Tap vs a single point, which amounts to a paltry .7-1.1% of threat in every parse I have done. 3 points is worthwhile, but impossible to get without dropping Toughness, which is a laughable decision until they implement a tankable fully spell damage boss of difficulty.
Talent Calculator - World of Warcraft - That is the maximum single target threat spec I use that still retains the "core" tanking survival talents -
Talent Calculator - World of Warcraft is what I use for 5 mans, 10 mans, anything else with trivial incoming damage.
I in no way advocate the second build for progression with any current encounter however... it's a single minded TPS/DPS spec built to roll content rapidly.
Edit - I didn't actually answer your question. I don't drop anything, I traded a Mark of Blood point for it. I always have Necrosis as I find it to be an excellent talent. In my build right now I think I am playing around with Corpse Explosion for heroic dailies.