"Great. Never hurts to have a marketable skill, right?" Being able to bill himself as a personal chef with a Stark on his resume my help Temugin move up in the world - into the hands of gentler masters, and perhaps eventually to freedom.

If his value is in a specific skill, it might also keep him from being purchased for other, less savory uses. Tony wouldn't feel right selling one of his slaves knowing that they might end up being purchased as little more than a warm body.