I agree with the notion that a warning shot indicates an unwillingness to injure or kill another. Many would think that to be an admirable trait...
Worse than wasting ammunition, a warning shot gives the intruder time and motivation to do great bodily harm to you or your family.
The sequence from firing the warning shot to taking another(?) or a "stopping shot" requires that the defender wait to see whether or not the warning shot had the intended effect. That gives the assailant time to press an attack -- especially if he has the right mindset.
In my view, the decision to fire a warning shot must be made well in advance, with a thorough understanding of the consequences of it not having the desired effect. To be sure, alternative scenarios need to be considered from the intruder being within an arm's length of you or your loved one to being at the other end of the living room and obviously unarmed.
Bottom line: if deadly force is authorized, your weapon displayed and pointed at the perp is sufficient warning. I'm not even sure that words to the effect of "Leave Now" make sense -- particularly if one is not good at the growling command voice under very scary and stressul situations.