When you enslave a mage or similar expensive unit, you basically get an ordinary unit with very high upkeep. Place these in the frontlines ... (unless you plan on GoR it).
And it should be noted that the game has no memory - GoR an enslaved former HoFer, and he won't regain his rightful place. Experience is retained, apparently the kills zero at enslavement. Guess there's innocence in ignorance after all.