I'm sure there are plenty of cases where TBN is superior. Personally, I've found the ability to heal myself and another player is huge. It doesn't cost GCDs directly (like Clemency). There are rarely back-to-back huge damage hits that would mean the regen-like effect of having to wait 4 GCDs for 4 smaller heals is ever a big deal. The way I see it, if there's a shared tank buster, the DRK can shield either himself or the other tank. Each tank could pop one CD, then apply Nascent Flash to the other tank - 4 hits later, back to full. Healers keep on DPSing.
I don't raid, so there are probably merits of TBN that I don't appreciate. It really sounds like splitting hairs to me.