Jeepers, fellas. Such hostility...
The problem is that a number of things are all true at the same time, and some of those truths work against each other at the extremes:
Slower bullets out-penetrate faster bullets, sturdier bullets out-penetrate softer bullets, bullets with more energy out-penetrate bullets with less energy, softer bullets create better wound profiles, harder bullets create deeper wound profiles, etc, etc.
You can play the same game with external ballistics as well.
A rifle bullet that penetrates 30 or 40 inches but creates a calibre-sized wound channel is a terrible bear defense round (for obvious reasons); likewise a bullet that creates a 12 inch wide wound channel 3 inches deep (again, for obvious reasons).
Research from Fackler suggests that 12ga slugs penetrate reasonably well, although (interestingly) not with nearly as much penetration as that from a 170gr .30-30 Win (Fackler, again). Furthermore, rifled slugs do not expand well, if at all, and as they're moving at not much more than pistol-bullet speeds, they produce very narrow wound channels (~.70 Cal), albeit very deep ones.
But put all the terminal ballistics nerdery aside for a moment to examine the platform itself: you would be hard pressed to find a more appropriate short-range defensive weapon than a pump action 12ga (or a good semi). Cheap, reliable, easy to maintain, easy to procure ammo for, easy to operate, easy to train with, easy to find safe venues to train at, light, compact, reasonably high capacity ... The list goes on.
At the end of the day, a defensive weapon, from the perspective of the user, needs to be accessible, reliable, capable of being used confidently and competently, and reasonably adequate for it's purposes in a terminal-ballistics sense. Like any other firearm, if you choose a projectile that creates a broad and deep enough wound channel, and superimpose that wound channel over a part of the target's anatomy that is essential to the target's health and wellbeing, then you will have succeeded.
The 12ga shotgun is more than adequate for bear defense. There are many other weapons that are also adequate.
And now no one needs to be touchy about the subject.