Although certainly it can be explained, this isn't about can a reason be derived for why something is happening, almost any occurance in a fantasy game can be given some explanation as to why it is occuring. It is more an issue of should you have to deal with them. If you just beat them in the field, from a gameplay perspective you've incapicated them for 5 turns. The reward for successfully defeating a champion is that you don't have to deal with them for 5 turns. The penalty is that you've lost the use of your champion for 5 turns and for non-sovereigns they get an injury (and the flavour text for Missing an ear is awesome). Having them available to defend the city, defeats this design decision in my view since when on the offense I'm attempting to breach their fields armies and take a city/territory. If your objective solely to defeat a field army (i.e. fighting a defensive action) in which you don't care whether the champion is in a town or not. It seems odd to me and undesireable. In my view, the penalty should be that the champion is fully lost for 5 turns.
To summarize, under the current system:
Defeating a champion while on the defense
- Enemy champion is injured
- Enemy champion/soveriegn cannot be used offensively for 5 turns
- Don't have to deal with champion/sovereign for 5 turns
Defeating a champion while on the offense
- Enemy champion is injured
- Enemy champion/sovereign cannot be used offensively for 5 turns <--- this is rendered moot
- Quite probably need to fight a weakened champion immediately
