There is a point. When you focus, your aperture will be wide open to let you have full view. If in completely manual mode, the aperture will be manually adjusted which results in dimmer light for focusing.
No, it won't!
To be sure, I just put on a manual KAF lens (SMC-A 85 f1,4). If I set the ring to "A", and the left wheel to "M", I see the chosen time and aperture in the viewfinder, just as I should according to the manual. I also get the information wether this setting will give so or so much under/overexposure. I can change the time and the f-stop with the thumb-wheel on the right side of the body.
Then I change the aperture ring to f-stop 16. In the viewfinder I then see the chosen time but not the chosenf-stop 16, no exposureinformation and full view. If I turn the on/off button to DOF preview, i get f-stop and over/underexposure information. Just as the manual says I would with a manual lens. Of course then the camera stops down to the chosen f-stop and the view gets dimmer, but that is just what should happen, even with a DA lens.
When I change to the kit DA lens, everything works just as it should according to the manual.
All this done with the menu set to "allow aperture ring". So the mystery remains: what is the point in setting it to "don't allow"?