I'm sure you recall Optimus loosing his almost a year ago (In DOTM), but I actually came across a picture from that Inspirational Pics thread saying "Bad news, Sentinel. I'm left handed." I know that's a joke and all, but what if it's true? If he is left handed, why would he need his right arm? Yea, it would be better to have both arms, he'll probably have a hard time transforming with one side of the truck missing, but putting that aside, he doesn't need the arm.