The funny thing is. Demo is actually better at killings those adds on mythic krosus compared to affliction. Yes it is a bigger waste for Demo and they lose some sort of long term dps compared to affliction, but the speed of killing adds is faster with Demo.
The demo player in my guild has quite decent add damage although I am a way better player. I feel like demo is better at killings add, but affliction does profit more from those adds, because of WoC stacks for example. And affliction does get the soul shards back when dying adds are afflicted by UA.
I agree though, both affliction and demo are tunneling speccs that perform best when purely tunneling one target