tbh the damage between BB and HS is meh so if there are more than 3 adds up just use blood runes on BB, and yeah you can switch target to DS/RS ect if needed though with vegence and 500% threat shouldnt really be needed to much - most aoe packs have a 'main' target anyway rest is just cleaves / multidotted
Know the pack of mobs you are fighting, mark the one you want killed first with a skull, attack that one while using blood boil to maintain threat on the others. If you lose threat on one of the other mobs just taunt it back and throw a death strike at it.
The only problem I usually get is if you have a ele shammy spam chain lightning as soon as you pull.
I tend to just use HS on 2-3 targets after getting up BB debuff and spreading diseases. Everything higher than that is usually BB spam with some HS thrown in if I feel like it but it's really not needed unless ya have whacky aoe burst dps with you.
I think HS is purely there for more damage for us Blood DKs atm during Hc content and serves no real other benefit other than single to 3 targets.
Unless you're worried about AoE threat just HS. You get enough Crimson Scourge procs for D&D and BB. Other tanks are better than DKs at AoE threat anyway to just focus on the add you want to kill.