[Lolzytripd]

It also bothers me that an expansionist Empire best compared to Rome, just executes usable prisoners.

Rome was heavily supported by prison internment slave labors. Strong prisoners skilled in fighting would be put into prisoner Legions and sent to fight as the first wave, if they lived long enough to fight of their debt to the empire they could be set free. ( one current campaign could be an eternal fight against the snake men of the argentyle cavern)

Most characters are imprisoned for civil disputes blown up into larger crimes. They aren't like the droves of bandits and monsters that force the empire to be a heavy handed discipliner, most of these characters have proven them self useful Via Fighting of the incursions of BDP's and/or Fighting off the hordes of black beasts that attack the empires lesser fortified towns.