I think this topic is quite important.
First, giving (serious) injuries to heroes can help balancing the game which -after a few games now- seems to favour heroes.
Secondly, injuries are fun and add a nice touch of customization to heroes.
. A nice way to remember your miscalculations in tactical combat.
As a fallen emperor, I will definitely use my 'beyond repair heroes' to cast the paragon spell or something like that (once the UI let' us choose someone else than the sovereig as the caster), and let them be old crones to defend the cities for when their adventurers day are long gone.
This is something Dominions 3 does very right IMHO. Weak heroes or now useless heroes become researcher in a castle. Also, holding some statistics about how many foes heroes have slain would be cool !