This has been numerous times and usually reflects what a user has on-hand or laying around. For awhile there was contest or sorts and/or bragging rights of what some uers had considered a PC. They've become re-used older computers like DEC or IBM mainframes, furniture, old appliance(radios), toys, even machine shop laser cut boxes. It's only the imagination that allows anything that re-uses some cast-off that gets a nod from a user to build upon. As stated to you, one big reason, most PC cases are "metal" as they ****** or allow fire protection as a side benefit. Another is that its yet another product sold when "customizing" is part of the setup. I actually haven't seen or touched that many novelty PCs, but if placing one on cardboard or kitchen table is one, then I've done this many times, then mount in PC case. The most useful or older PC was prior to laptops and generally you may think along the lines of Compaq "luggable" which wasn't even around. We build a test bed to load diags into mainframes using a 8in floppies and tape in order to have low-level operation to zero-in on damaged portion of a mainframes, that was in the early '80s. Only 1-ever made and once the word got out, everyone wanted one. That's was out of hands expect for the engineer that made drafting schematics and provided it to other depts., what became of it I don't know. But a small company called Irwin introduced a product based on a 1/4in. tape device that lead to others and then Iomega brought them out. (from fuzzy memory, maybe we shouldn't had written it down)
tada -----Willy 