Earlier quoted context omitted.
This is why I like the "open case design." Your "PC" is just a frame to hang components on, with nothing to enclose it. And the nice thing is these days people care a lot about quiet computers, so you can have an open frame case with three GPUs that is just making a not-unpleasant whooshing sound at full bore.
This exposes the components to more dust and potentially static shocks as a result. Cases are also designed to encourage airflow over specific components, so being case-free might not result in improved thermals. But it is certainly more convenient for tweaking your build.
Open air certainly is a choice and cases do function to better guide airflow when designed and built in correctly