Version: (using KDE 4.0.82) ...because if the panel is covered you have to play tricks to get to the one behind it. Or provide selection which one user would like to select (to configure it). Of course the effect of configuring is up to the user, so if in effect the panels will overlap -- fine.
This is a regression from KDE3. In KDE3, you can put multiple panels on the same edge of the screen, and they do not overlap - they stack vertically instead.
*** This bug has been marked as a duplicate of bug 165792 ***