Its just a case of design over funtion, if Apple had its way there would be nothing on the right side, it wouldnt be hard for them to make a few usb ports on the left if they wanted too.
Actually it's due to people constantly complaining.
Original: ports on the back, but it's a pain to plug in and remove anything with the laptop open. some pcs are still with this design.
v2: ports both sides, people complain why ports are not on one side, cables out of both sides are messy for some people
v3: ports on one side (partly due to the unibody thing and also due to stress testing), people will complain more.
I have an alum MBP 17" with USB on both sides. I think that works.