I'm not sure if you're asking why there should be disabled items or how to disable selected items. Why is because there may be situations where menu items don't apply and you don't want a user to select them. As to how, if you have a menu ring with items a,b,c, and d. If you want to disable b, this is element 1 in the array of elements and to disable it, you create an array with 1 as the only element and wire this to the property node DisabledItems (LV 6 and up. Not sure about name if 5.1). On the Front Panel, you can simply select the item you want to disable and then right click to enable or disable it.
