I once threw a computer into a case for a sawzall. installed standoffs, hard drive cages, powersupply everything. looked like a grey plastic tool box with a couple holes for wires. I used this as a mobile testbed for a couple years, then donated to my computer school.
I have a friend who only wants the motherboard trays, that is his case, hanging on the wall a bare motherboard in a tray.
as long as you can make sure the motherboard will not ground/short itself out, and put it where is will not be killed by the elements, I think it would be ok.
As to benefits I can only think of one for a technician. Having a system breadboarded and ready to test cards, RAM, CPU's it is beneficial for a tech. at home I can see no reason to do it.