Well,
easy question. Sry for my poor english
simulationcraft.org/430/Warlock.html
If you look at the current T13_H sims, you will see that Demo wins the dps race with 52.300 dps, 1,7k more than destro/affli.
13% of the Demo sim comes from using Shadowflame and Immolation Aura, that's worth 6800 DPS.
So, apart from the "heavy movement" sims - where the specs are getting closer and closer.
If you can't stand next to the boss, why should you specc demo?
and
If there are 2-3 Enemys - why should you specc demo? BoH is worth 15% dps, affli multidot even more.
Isn't Demo limited for fights with heavy AOE Situations? I would prefer Destro/Affli as it gives me more freedom and less rng.