Biggest thing people often don't realise, especialy after cleaning out a few years of accumualted dust in one recently having overheating issues, a well designed rig utilises an enclosed case's design for airflow. Without the case, it surprises people when their harddrives or other oddball components on the board will start to fry. A good do-it-yourself computer person knows you need a case for protection and to have a effecient, effective and minimal system cooling (use a couple fans for really good circulation rather than like 12 all over the place and in each dirction because you're clueless).
Personaly where I live, dust is a fpita issue. Next rig I build I'm gonna have all my intakes go through a filter.