Even a high value caster would be a dubious proposition. Having a powerful caster with low loyalty in your capitol is a risky proposition. What happens when in the middle of a fight, someone gives a better offer. Maybe like a chance at revenge for their fallen side or a chance to attack an old enemy of their former side. Something that could appeal to their loyalty to the fallen side.
You also don't know if the caster in question had a part in the fall of their former side. Why are they safe in the MK instead of disbanding/croaking with the rest of their side?