You dont rest your hand on the shifter because it does not allow the synchos to sit in the desired position. If you add any lateral pressure (left/right) to the shifter you are in effect always pushing it a little bit out of gear. This is not as noticeable on our sticks, which sit upright. However on cars where the shifter is tilted (like trucks), if you rest your hand on it, the gear that you rest it in will become worn.
I knew a woman who rested her hand on third in her truck, because this is the gear she used for her rural commute.
Eventually third gear failed.