Well the following are my opinion based on my training, shooting experience, and legal experience.
First, I start from the premise that any time I would fire my HD weapon at another person it is because that person presents an imminent threat of death or serious bodily injury. If they do not then I am not legally justified in resulting to lethal force. As an aside the legal justification behind "castle doctrine" laws is that one that has broken into your home is presumed to present the type of threat described above.
If someone is an imminent threat of death or serious bodily injury then my proximate goal is to stop that person from being such a threat. What will cause them to no longer be such a threat? That is hard to say in every case. There in essence two ways in which people are stopped from being such a threat. First is a physiological stop. This is in essence when the person who is a threat chooses to change what they are doing or their intentions. Any number of things could cause this to happen. For example, the bad guy sees a gun and thinks better of his course of action and flees. Someone being shot and at that point giving up would be in this category. Someone desisting their offending actions because of pain, injury, etc would all fall into this category. The problem with psychological stops is that you can never rely on it happening. What will be sufficient to cause one person to no longer be a threat may not even phase another person. I am personally acquainted with someone who was shot during an incident and literally didn't even notice until the excitement of the event was over. Another recent example of failure to produce a psychological stop was the video that circulated of the gentleman who opened fire at a meeting and was engaged by a security guard. The shooter was struck multiple times and continued his attack and even after being shot more continued to return fire at the security guard while he, the attacker, was on the ground.
If a psychological stop cannot be relied upon, and there are a myriad of examples of it being insufficient, then we must turn to the second way in which a threat can be stopped, a physiological stop. Simply put this is when the person is rendered physically incapable of being a threat. Common ways for this to occur are, unconsciousness (say from loss of blood), death, damage to the nervous system, etc.
If a physiological stop is a defenders goal then that defender needs something that can produce those results. It is worth remembering again at this point the nature of the threat, it is an imminent threat. Thus, the defender needs not only a physiological stop but he needs it ASAP. To get this kind of stop projectiles need to either hit the central nervous system (about the only way you are going to get an instant stop) or alternately reach the major vital organs and do sufficient damage to cause sufficient blood loss to stop a threat.
With that in mind bird shot is simply a very poor choice for such a situation. It has a tendency not not reliably penetrate to major organ or the CNS. That is true in a full frontal shot, in the real world a defender is not always presented with a square full frontal shot. A projectile might first strike an arm or something else that is in the way. The wound channels that bird shot produces are also less than ideal for meeting the above stated goals.
I really cannot think of a good reason to have bird shot in a SD gun. Even if there was a reason I would not mix the tube. A mixed tube is not a real good idea generally. If I am going to fire my weapon I want to have some reasonable idea of what is coming out the end and how it will perform. In a SD shooting incident I do not believe it is reasonable to expect one to be able to count shots or keep track of the order mixed ammo etc. People seem to have great difficulty doing this under what I imagine are less stressful events than a life or death situation.
A common justification for bird shot is fear of over penetration. This is moot if you are mixing in rounds of buckshot. I also believe it is poor logic. Their are better ways of dealing with that concern while still having a tool on hand that is better suited to being able to stop an imminent threat.
I could go on but I am sure I have written enough to bore anyone reading this. In sum, I see no upside to the bird shot. I see it as a liability and an even greater one when it is a mixed load.