I can tell you that in malpractice cases, where the plaintiff wants to make the doctor look like an incompetent practitioner, things like GAF do matter.
"Dr, you diagnosed the patient with a GAF of 45. Can you state the exact criteria for a GAF of 45?"
"Doctor why not give a GAF of 65, or 35?"
So when a doctor gives a GAF, and can't state an exact reason why they did what they did, they look like an idiot on the stand, and lots of doctors give a GAF as if they're critiquing abstract art.
If all doctors did the GAF as accurately as possible then it would matter. During residency, no doctor I knew took it seriously other than to put a number down that would back up what billing companies were looking for, and that appears to be the current norm.
But when I hit fellowship, my PD mentally slapped me silly (in a good way) to make me take those numbers seriously. In hindsight, I thought about it, and remembered that no one, literally no one made me do the DSM GAF the exact way it was supposed to be done until fellowship. Yes there is an exact procedure on how to do it. Don't believe me, read it in the DSM. I never read the DSM in a manner where I read it page after page, starting from beginning to end until fellowship. I just used it for reference. While I did read the DSM scale, I never actually did it procedurally the way the DSM tells you how to do it because no one in residency told me to, and I noticed I was actually putting more effort to be accurate with my GAF number than my attendings.
If everyone did it the way the DSM tells you how to do it, the number would matter, but hardly anyone does.
I've given a GAF of zero much more after fellowship because my PD got me "straightened out" so to speak. Zero is not worse than a GAF of one. Zero means you don't have enough information. I've had several people in medical records call me up and explain to me there is no zero score. I tell them to double check the DSM, and then usually about 15 minutes later they call up and apologize.
I agree with the above, however, that there can't be 100% accuracy and reliability between psychiatrists when doing a GAF< but if psychiatrists did it the right way, it shouldn't deviate by more than 10 points, because the criteria in the scale ask for things that psychiatrists are supposed look for such as the severity of the symptoms or the person's ability to function in society.