So I wouldn't say brain damage leaving you alive but afflicted is as "extremely rare" as you make it out to be.
Ya, there's just one little descriptive chunk that will clear it up: brain damage from fighting(or work accidents) should be mostly lethal.
I think that before we accept the blanket statement that brain damage should be mostly lethal, we should have an expert chime in. The brain's very modular, and location of trauma has a huge effect on what happens. I mean . . . brain stem gets damaged, you're probably toast. Hypothalamus gets damaged, probably toast. Anything having to do with maintaining homeostasis, and your outlook's not too rosy. Obviously, any attack that immediately immediately pulverizes everything in your skull will encompass these areas which are vital to minute-by-minute function.
I'm reasonably sure, though, that anything causing damage in other ways--glancing blows by blunt objects, concussions, slashing injuries by smaller weapons, and especially piercing attacks--maybe things like spears assuming the area affected is limited, but especially bolts and arrows--is going to cause a variety of behavioral issues.
I'd imagine seizures would be one of the most common manifestations, on par with blindness and loss of other senses or motor dysfunctions. Probably more likely than, "Hey! I like Olivine!", anyway.
edit: and by "an expert," I don't mean me.