You have to think about work load for the people that build the cases. If they are extremely easy to put together (click click click) then the technicians probably just shot them together willy nilly - like you would do if you worked there
I have a ThermalTake Xaser II and I have 2 fans at the front blowing into
my case, 1 at the side blowing in
, and 2 at the back blowing out
. Then I have my PSU, CPU fans etc.
But the correct configuration is the side blowing into the case, and the back blowing out; working in conjunction with the PSU - and giving an overall healthy airflow.