Ik, ik this is quite a stretch, but just take a minute to think about it, If you are a Atonement/AA build Disc priest, in which case Atonement is a fairly hefty chunk of your heals, meaning your consistently throwing out Holy Fires and Smites, which are technically damaging spells, wouldn't those have a chance of proccing the *Duplicate Spell*? Im making the assumption that duplicate spell means the spell is going to hit for say 20k then immediately after hit for 20k, now a 20k smite hit will usually throw out a good 24-25k heal on someone nearby in need, so wouldn't having a chance at throwing out basically a second "free and instant cast smite heal" be a nice thing to have, I mean it all depends on the proc rate of this thing, but like I said this is all speculation. I realize the hit almost makes my argument invalid because of the glyph of divine accuracy, but there is a thing as reforging, now I personally would reforge 150 of that hit > spirit, although its not ideal that's still a hefty amount of spirit you can pull from that weapon, giving you a "reasonable" spirit/haste healing weapon, both of which are very beneficial. As a disc priest, or at least on my disc priest I also feel I don't need nearly as much spirit as say a holy priest so the lack of spirit on Dragonwrath wouldn't be as detrimental to a disc priest as it may other healing classes. I mean let's face it, the 150 Int you get from sockets and bonuses is pretty hard to pass up :/
Alternatively, if your a Disc/Shadow priest, reforging the staff's hit > spirit wouldn't hurt your Shadow Gear set either, because it would be as if you never reforged out of any of that hit, so you could literally leave that staff unchanged and be able to switch in and out of it from disc to shadow, without any re-reforging needed at ALL!
Just something that crossed my mind, I'd love to hear what you guys think!
One thing to clarify, I am in NO WAY saying that this legendary should ever ever under any circumstances be given to a healer over a caster DPS class, I'm just saying if you at some point were able to get your hands on this, would you find it viable as a disc priest.