When you see someone channeling a Felblood Initiate but havent tapped it yet, when the channeling is almost done, you could tap it and kill the Emaciated Felblood that it turns into, therefore not wasting any demonic blood.
Pros:
Don't have to kill any Wrath Heralds for Demonic Blood.
Faster than having to do it yourself.
Don't have to get beat on while an elite is hitting you (only when they aggro and you have to either run away or use the Fel Siphon)
Cons:
Slow if there aren't alot of people doing the quest.
May cause the opposite faction to gank you out of anger.
May cause the same faction to talk shit about you when you in trade chat when they see you.
So yeah, thats bout it if you guys didnt alredy know about it.