For 7 years my desktop has been literally just a motherboard sitting on a cut-up yoga mat on my bookshelf. [1] The SSDs are piled next to it. My work PC is zip-tied to a milk crate. Don't let people fool you, you can be pretty creative with your definition of "case".
It took me at least 5 years to get a new case, much to my wife's annoyance.
It was fine. Swapping SSDs was easy. >.<
I used to just have it directly on the wood, which was fine too, but then I put it on top of an upside-down, powder-coated-metal IKEA drawer thing (so I could put the power supply underneath for more shelf space). My fiancee was worried it might get scratched and conduct at some point, so we added the yoga mat.
Not at all, it's plastic!