Measure your temps under load using HW monitor with the side of the case on and off, there may be a significant difference, there might not be if no one really designed the case with good cooling in mind.
Not necessarily, the way most cases are set up, air flows in the front, across all the components and gets sucked out the back, if you have the side of the case off you dont get air moving at any significant speed and will often just end up with it pulling air in through the side and shooting it out the back of the case without passing it over the components.