and i believe that is the ONLY time you can cast it, correct?
since after combat damage its no longer a blocked creature?
509.1h An attacking creature with one or more creatures declared as blockers for it becomes a blocked creature; one with no creatures declared as blockers for it becomes an unblocked creature. This remains unchanged until the creature is removed from combat, an effect says that it becomes blocked or unblocked, or the combat phase ends, whichever comes first. A creature remains blocked even if all the creatures blocking it are removed from combat.
While it is certainly the most common time to cast smite, it is not the only time.
If I steal a hundred dollar from a loot of one thousand, people might notice;
If I steal a hundred dollar from a loot of one million, I might get away with it;
If I wish to steal even more and still go unnoticed, I need to make the loot bigger.
Now you know why taxes always go up.
Looting: ''the plundering of public assets by corrupt or greedy authorities'' (Wikipedia)