Interview with Jeff Kaplan.

[Warlock]

I can't see this being anything but a shard, let alone popular.

Such a set is just so marginally useful (at least for PvE) that it leaves me speechless that people think this is a serious idea. On the rare fights where I might want to have a hybrid set of gear, I can mix and match pieces to create my own. And I will already have those pieces in my inventory.
Now that would actually help (at least the Smite part, no one casts holy fire soloing except for the opener). But then you'd be tied to some overpowered set bonus forever, kind of like how 3 pc. transcendence was.

Druid tier Feral gear already does something similar (they all include caster stats which are of zero value to a tanking Druid and limited value even to Cats; fortunately the caster sets are pure). Hopefully the 'free damage' gear will be more attractive to healers than the Feral sets are to Ferals. Something like 'replace all +healing with +damage; increase coefficients on all healing spells so that +damage is as effective per item point as +healing was' would be optimum.