There is no advantages of an open case to anyone but ppl using a test bench who swap out parts in a continuous cycle. Dust is not an issue except for testbench or open desktop designs that aren't running, where falling dust can settle directly onto the components. Dust has a hard time traveling sideways, so desktop towers don't suffer much if any from falling dust as the gpu/cpu coolers and case fans create enough turbulence to keep any falling dust away. If you have a desk fan blowing directly into your tower, well that's a separate issue.
Humidity. Hah, non issue. There's 0 change in water density from air sucked into a case by intake fans, and open air cases. Air is air, it's all the same. So is the humidity.
Case temps vary...