I hope he used some -really- good screws, because if any one part of that computer comes off the wall he's going to wish he'd bothered to install a case...
When attaching things to drywall, its not the size of the screw that matters, its where you screw it. *giggity*
The idea that you need a computer case is all part of a conspiracy by PC manufactures! Free yourselves! Let it all hang out!
you don't 'need' a computer case, it simply makes it easier to transport/protect the sensitive components from nasty things like gravity and cats.
What if cats are just trying to save their human minions from the eventual AI singularity? Or themselves? Hmmm...
also using a case with fans creates airflow, because stagnant air is so horrible at transferring heat.