I cant see any reason not to be kind to your sub. I mean the main reason a sub is willing to submit to you at all is if you treat her right. If you are just a harsh man and abuser... who in their right mind would want to stay with you at all?
Yes some people gets off on it but not any sub I would want to have.
A good Dom/me cares for his/her sub/slave. Thats my stand on this.