I went into the PTR (I understand it's not the same without the artifacts) and tried all 3 specs. All seem nice (I especially like demo), but only destruction seem to actually be the best choice for most raid bosses. That is because it has burst and effective cleaving/target swapping, while the other 2 specs need a bit of time when they switch targets.
Maybe if someone specs demo with implosion, hand of doom and the eye laser monster, there could be some more effective low-health-add killing.
I wouldn't have any problem with playing with 2 specs and switch to whatever is needed, but with the artifacts, it seems like we will have one stronger (more leveled up) spec than the other. So, if I main demo and have destro off-spec, if I have to spec into destro for a boss, it might feel like I am not playing to my full potential, because it's not as leveled up as my demo artifact.
I don't know, any ideas?