When you talk about intruders, I'm assuming you mean those belonging to the human species...for those I favor my Model '97 loaded with No. 6 shot. Remember, in this day and age you will be required to explain your actions. If it looks like you were having too much fun, things could go against you. Nobody walks away from 6's at 10 feet. If they are outside the house you better be real careful about any action you take.
The legal ramifications and perceptions are the reasons I choose the tools I do. I use similar equipment to that chosen by law enforcement. My ammunition is chosen the same way.
You're right, most people don't walk away from 6s at 10 ft. But, they can do some mischief before succumbing to wounds.
Unfortunately, in my house I have the potential for shots over 10 ft, my longest shot is a little over 35’. I use something that will be effective at the greatest distance in which I may have to take a shot. My chosen gun/load will keep all of its pellets on a silhouette target out to 21 meters reliably. In the SD/HD classes I've taken along with the training I've had for a PD and Army, 21 feet is considered a "keeper" for an armed assailant, in that at that distance an attacker has a good chance of reaching you before "the threat" can be neutralized. Now that is with a blunt object or edged weapon, a firearm is a whole new ball game and the distance is of course increased.
The only reliable way to stop a human attacker instantly is by hitting 3 locations, brain, brainstem, or central nervous system above the waist. Anything else and they can still pull a trigger until they bleed out. As you well know, animals can soak up a lot of punishment, I’ve killed elk that didn’t even register a hit and went a ways before going down, and how far have you had deer run after heart/lung shots? An animal running is one thing, a still functioning knucklehead hopped-up on drugs with loaded gun in my house is another. Everybody worries about over-penetration of his or her weapon, but the potential is for two (or more) people with guns. I know the fields of fire within my house, IE who’s bedroom is where, the other guy doesn’t care. I want that threat neutralized ASAP, so I use a tool that has the effect of hitting with eight 9mm rounds simultaneously. .
BTW in two tactical shotgun classes, the birdshot question arose and was not recommended. This is due to the lack of penetration. The FBI penetration tests call for 12” with handgun ammo, this takes into account outer clothing and shots at “bad angles’ IE through limbs etc. That's 12" regardless of distance. Autopsies are great, but they only take into account the ones that died.
Here’s a decent article on shotgun wound ballistics from the Western Journal of Medicine of special note is page 155 which deals with mortality rates based on distance:
http://www.pubmedcentral.nih.gov/pagere ... dex=3#page
The bottom line is, we’ve all got to do our own risk assessment, I value penetration above the potential risk. If I can all help it, no intruder gets within 10 ft of me, period. I think that bird shot is highly effective when used to defend against attacking birds