This is mostly useable on a Pve realm. Normally if you attack a mob and someone else kills it for you, you wont get full exp. But if the one who kills it is from the opposite Faction you will still get 100% exp! Think of a lvl 40 ally running around attacking mobs while a lvl 70 horde kills the rest of their HP, it would be really fast.
If this is a Repost then sorry