It's just a contributing factor. No one is ever going to try to make the claim that the head injures are the direct cause. Well, no one who is reasonable anyway. Short of some condition caused by a specific gene, just about every medical condition is based on a collection of factors. All those plates of french fries didn't cause that heart attack, but they definitely didn't help.
Too often people want to say that since something isn't the main cause, there's no reason to deal with it. If we can make steps towards correcting this problem, then we can move on and go after those other causes as well.
And yes, that emotional drop off you mentioned probably does have a big impact on certain people. It makes you wonder though why it hits some so much harder than others.
Wait, did Hamdan actually get his revenge?