When I put together a raid I'm not looking for an Shadow Priest to heal. Hes ranged DPS, just the same as a Warlock or a Mage. Hell, I'm sure my Warlock does a lot more healing per fight thanks to Healthstones.
My point is that if Blizzard cared about restricting rezes to healers then they would've made it a talent deep in the Holy or Disc trees. SS is, technically, already a rez anyway it just has to be used preemptively.