A few thoughts:
Brain shots are tricky. In bears, it’s not really because you can’t reach the brain with the bullet – you can, easily – but because the brain case is unexpectedly low, rearward and narrow when viewed from the front. Bears also have a very irritating habit: they move their heads around a lot – true of all things with both heads and necks – and although they tend to stay focused during a charge, they move fast and low, and that makes a moderately-difficult shot much harder. This is why the vast majority of people are better served with an area-effect bear spray that doesn’t require you to be able to hit a target the size of a mango that’s bouncing towards you at 40 MPH: it simply gives most people a better chance of stopping the bear, or at least rendering it temporarily less capable of doing harm than would be the case with an attempted shot to the central nervous system.
For those thinking about a heart shot: good luck. Yes, any sufficiently-large, deep-penetrating solid that’s shot from almost any angle will take out the heart, lungs or the big arteries connecting everything, but there’s usually a rather uncomfortable time delay between those shots landing and the bear expiring. Several seconds, in some cases; that’s more than enough time for a bear to acquire you and begin a thoughtful remodeling of your existence.
Other shots are easier to land, but simply unethical; they are, however, extremely easy to accidentally achieve during a crisis. If one is having to weigh human life against bear life, then yes: do whatever is necessary to preserve human life, even if ethics must be temporarily discarded…but by carrying bear spray, that ethical choice can hopefully be prevented, and the life of the animal spared.
All this being said: if one wishes to back up bear spray with a sidearm, then do so, by all means. That is a right that we all possess, whether or not we choose to employ it.