For historical info, I’ve judged armor since ‘95, diorama categories used to be within the major categories they fit in, Armor dio category in Armor, Aircraft dio category in Aircraft, etc. I can remember judging dios at Columbus in ‘97, I don’t remember when it was changed to a stand alone category. I can not fully explain the reasoning but I do know in armor we end up moving a good number of entries from other armor categories into the dio category due to these rules;
E. Bases – in General. Bases will be allowed in all categories and will not be considered in the judging, except in Small/Large Composition (vignette/diorama) categories. A base may be a piece of undecorated wood, plastic or glass or it may simulate the natural surface on which the prototype would be found. However, nothing other than that basic surface may be used. Aircraft and military vehicles may rest on simulated ground or paving, and aircraft that need beaching gear or dollies may be so equipped. Ships may be displayed in water (no dry docks). The base must not be the predominant feature of the entry and must be of a size proportionate to the model. The Contest Chairman and Judges reserve the right to exclude oversize bases. The model may include primary crew figures. (See Rule IV-3B for limits for Military Vehicles.) The addition of any other figures or equipment outside or not attached to the model (e.g., support equipment, shell splashes, or buildings) will make the model a vignette or diorama, which must then be entered in the proper diorama category. (See Rule IV-1D.) So, if you want to compete in a single-subject category, don’t make a Vignette to see how far you can push the boundary!
B. Bases – Military Vehicles. Any AFV model that is displayed upon a base may have ‘basic’ groundwork; e.g., dirt, grass, roadway, a low stone wall, etc. No part of that groundwork may extend taller than the topmost portion of the body of the model; i.e., no trees, buildings, structure, chimney, water-tower etc. Any vehicle entry that has more than basic groundwork will be defined as a vignette (see Rule IV-1D) and will be transferred to the appropriate category for judging there. Any model entered into an AFV category that is displayed upon a base, permanently or temporarily, may have no more than two (2) figures in total and the figure(s) must be a crew member; only one (1) figure may be affixed to the base off the vehicle; a second figure must be affixed to or within the vehicle model. If there are more than two figures total, the entry will be defined as a vignette, and will be transferred to the appropriate category for judging there.
That puts a strain on the dio judges as there never seems to be many volunteers to judge that as a stand alone category. I imagine this change was in part due to the reality over the last few contests of the lack of judges to do the job. I don’t see this as bad for dio modeler’s, judges will follow the rules and having an armor dio judged by armor judges and aircraft dios judged by aircraft judges seems like a plus to me.
If it doesn’t work I imagine a change in the future.