Closing the building prevents people from seeing the interior and reduces play value. Open backs, while less realistic, allow great access.
I think the best set design can be seen in sets like the new Locomotive Repair Facility and the old "Cargo Center" from the 80's. These sets were hinged - and always had an open back, but depending on the orientation, they looked completely different.
> that set. the new grand central station is also attractive, but again they
> cheat you on legos by having an exposed backside of the building. make it
> realistic and enclose the building.