Also, consider that Divine Smite only matters if you hit something. Having to use DS before you attack means there's a real possibility you'll spend a spell slot to miss. Even if Larian makes it so DS will proc on your next *successful* hit, you still have to wait a whole round before you get another chance to use it. By then your other party members may be able to down the BBEG before it even comes back to you, and you'll have wasted one of your few spell slots for nothing.
It'd be vastly superior if Larian implemented it such that your spell slot is refunded on a miss. Otherwise it's decently likely you'll waste it on an opportunity attack. Or, as you say, the combat might be over by the time it gets back to you.
