No idea if you can use the trick more than once but common no need of infinity (now I'm sure you are from Marseille!
).
I know in general lost a battle is end game but apparently not in Demonis and I remember to have see in a review that that happen too if you die near a castle.
The point would be to keep an enemy troop in Demonis or close to a castle to keep open the possibility. Useless if you are trying to make a score but can be useful if you are playing slowly the game and tried different set of army. But that's the problem it's hard to say what you'll not use anymore but few weaker units in small number that don't worth the sweet.