Unfortunately, Power Word: Barrier has yet to make an appearance on the PTR, and it turns out that we may not see it at all. It's still a possibility, but that's the extent of it. I can sort of see why they would hold off on it, or even change their mind about it. Discipline has gone through a lot of changes in patch 3.1, most of it a really big numbers game. Discipline is a great tree, and while most players might not 'get' the damage mitigation/prevention thing, it's a really powerful mechanic and probably one they need to be careful with.
The combat effect of too much absorption is important to consider of course, but, I can see how it would even have a psychological effect on players. If a Discipline Priest's absorption became too powerful and it seemed like nobody was really taking damage, wouldn't things seem really, really easy? Even if the Discipline Priest was working their tails off? Malygos's Vortex is certainly not dangerous anymore once the raid has enough HP to live through it, but what if a Priest just popped Power Word: Barrier during it? You wouldn't take any damage at all.
There's something to be said about the psychological side of raiding, and I think that plays a role here. If someone sees they didn't take any damage, they're going to think things are too easy. If they're taking massive amounts of damage and someone is healing them through it, they'll be impressed. That was a lot of damage, and that Healer was on top of things! The Discipline Priest could do the same thing via absorption, but people wouldn't be as impressed. Their health bar wouldn't be bouncing wildly, they just wouldn't be getting hurt at all.
I certainly hope that we see the spell in patch 3.1, but I don't think I would be heart broken if we didn't. It would be nice to have an answer to the Holy Priest's Circle of Healing, but that in itself is a whole new can of worms. If we have Circle of Absorption, does that have a negative impact of Circle of Healing? We'll see how this all plays out eventually, I suppose.