Its effect is by its very nature conditional because in order to be compared to EHP, multiple conditions have to be met first, like the tank suddenly not getting sufficient healing for whatever reason but getting just enough passive healing bonus from the proc to survive the next hit in a very specific time frame. You can see this in
DarkenedHue's thread. In order for you not to get sufficient healing, it means your healers screwed up and in heroic raiding, you're going to die when that happens, Last Word or not. You shouldn't gear around those situations because the only thing that will realistically save you is a well placed cooldown, being a paladin or lucky avoidance RNG.