Thanks Narkissos. I was surprised when I heard the Catholics leave it off the end.
I've heard Christian pastors teach that this prayer is only a guideline and not to be prayed literally. I think this is done to get away from prayers that are memorized and repeated the same way every time, the way the Catholic church does. Those who teach this presume that this kind of prayer loses its meaning, since it can be made without giving any thought to it. They want to distance their congregation from that practice, and have their prayers be more spontaneous and personal.
So if the WT teaches this, I think it's done to distance itself from "traditional" Christian churches, primarily the Roman Catholics.