Sorry but I don't see any harm in this development. As has already been stated here, the existence a mental disorder does not relieve a person of legal responsibility.
More importantly though is what it means for drivers seeking treatment for their anger. If the disorder is in the DSM-IV-TR (the most recent version of the Diagnostic Statistical Manual of the American Psychiatric Association) then insurance companies will reimburse for treatment. This, of course, increases the number of people seeking and getting treatment.
Ask yourself, do you want drivers treated for IED? Or would you prefer they continue driving angry? It is very common for courts to order road rage drivers to take anger management classes. I think this is a good thing. The designation of IED as a distinct disorder gives courts one more way to deal with road rage drivers (i.e., they must get treatment).
I see no problem with it being a way for them to get help but it also gives the lawyers a card to play to get them less punishment. If they agree to get some type of help it could reduce if not eliminate their sentence.
I disagree that the existence doesn't relieve them of legal responsibility. I have seen where people who have epileptic seizures have had one and had their car kill an entire family. Do you think they got voluntary manslaughter or murder? No, but in my opinion they knew they had this problem and yet they went out driving with the knowledge it could happen at anytime and kill someone. Even more scary is they shouldn't have had a license with this condition anyway. I even know diabetic's who have many low sugar attacks and have blacked out many times driving, endagered their own kids, had car wrecks etc because of this yet they still can drive and nothing is done to them...
Sorry I don't agree or get it...