Well my thinking is this. Some one stop me if I'm wrong. only the moving object are rendered with motion blur on becouse if the none moving objects were it would make the exposure wrong. Just how the computer dose it. and / or it would slow things down even more. Some renderer's the bojects that move many times to get a 3d motion blur.
When you step in to useing motion blur with your sean, you must composit. Meening render you back ground Plat or animation and composit it with the animation of the moving objects. Render that part in a nother pase with alfa and put them togather. If your rendering out to frames and putting them togather with a nother app whitch is the best if you ask me.. you can eather use that app, or if it dosn't do the job good enufe for you, you can batch it in phtoshop.