Since we are all adults, I'll disagree with that... birdshot should be used for birds.
While I wouldn't want to be shot with it, birdshot is not going to do anything other than make a nasty wound. It isn't going to penetrate, and in turn, will not stop a threat. Plenty of people can attest to that in hunting accidents... just ask Dick Cheney's hunting buddy. 78 year old took a pattern of birdshot to the face within 90 feet (depends on who you believe, probably closer).
Or better yet, take a look at dogs that people have shot with birdshot. If you look hard, there is very graphic shots of a pit bull that some random scumbag put birdshot into its head. Most dogs can be blinded, but rarely is it fatal.
Buckshot, in comparison, has about two to four times the mass of birdshot pellets. A .33 sized ball will get into vital organs and make serious injury to them. Want to really make sure you inflict damage, a 12 gauge slug is anywhere between .69 and .73 caliber... usually moving even faster. Blood will leak out of such a hole pretty well.
Now, if you are going to suggest birdshot in a defensive shotgun... why not do less lethal? You'd actually have more chance of stopping your threat with blunt trauma of a beanbag to the head than birdshot anywhere. Even if you are close in, the pattern of birdshot spreads. It isn't like you are going to be hit with a straight ounce of lead, but something like 200 0.02 ounce pellets that will not go through fatty tissue. Only exception would be if you cut the shell... and you may have a Glaser slug like performance... but you really don't want to be cycling cut shells through a shotgun.
The main reason for a defensive shotgun is to stop a threat. Buckshot is my smallest load I'm putting in a defensive gun... and I do stop at 1 (.30 balls, but normally load 00/.33). If it is a bird or clay, then I'll use #8... which will do the job in that role.