ok, maybe I'm being foolishly optimistic, but why have a closed case at all?
So that you won't drop a metal coathanger onto your motherboard where it accidentally shorts two contacts together that carry moderately high power and makes a little 'pop' noise and creates a little blackened burn mark thereupon. Not that I would know from personal experience or anything.
In a similar problem, note that some old power supplies (anything before ATX) had somewhat dangerous on/off switch arrangements. The line would come in from the wall, then be connected directly to 2 wires that went to a powerswitch on the front of the case. Often the power wires from the PSU to the switch on the front of the case were a bit carelesssly routed and flimsy and had easily loosened covering where they connected to the switch. They were in close proximity to sharp pointy sheetmetal, and to people futzing around with screwdrivers and god knows what else. They carried the full 120VAC line voltage and current, so if you , say, jammed a pair of scissors in your open case just wrong on accident, or had loose insulation near the switch and dropped liquid on it, then you would be getting the full shocking effects just as if you had done the same thing to an electrical socket. A sturdy solid enclosed case would prevent many such accidents, so there is another reason to keep it closed. Probably less of an issue with ATX.
Also there is the 'faraday cage' effect of metal around your box to prevent leakage of all sorts of EM signals going off interfering with other electronics components. I believe metal mesh will work for this, though I am no expert.