I don't agree that decasing a PC fixes heat issues. The only true solution to heat problems is to remove the heat from the enclosure. If you have no case but no ventilation at the top and bottom of the cab, you still will have heat issues given a long enough period of time (assuming you have a wooden cab). If your PC case has a good cooling system (well designed air flow), you are actually going to be better off than just surface mounting everything without the case. I won't get into technical details, but it has to do with forced convection being more efficient for cooling than natural convection especially with a lower delta temperature between the ambient air and the component temperature (especially if you have poor cabinet ventilation).
If you force air through your cabinet (draw cool air in the bottom, vent out the top), then you probably would be fine without the case since the ambient temperature will be fairly cool (unless it's a hot summer day of course).