Well, keep in mind it isn't so much "if you have trouble staying alive" but having access to as many tools as possible to keep yourself at a safe hp. Picking a subspec for blood tanking is asking yourself what would you have more of, survivability (frost) or threat (unholy). If you don't have any threat issues (you shouldn't) then why would you not get more survivability?
Sure, you may only use Lichborne/Death Coil heals once in a great while when you have no other tank CDs available, but when that happens, and it saves a wipe, it was 100% worth having over some extra threat that ultimately wouldn't increase your groups overall rdps. Just my 2 cents.