I don't see any reason why someone would want to choose the default random behaviour over the consistent one.
Because it is not random ! Many unix environment system (GNOME, KDE, XFCE at least, Compiz i think too) automatically create window at the position of the screen where there is actually less window opened (upper left by default, maybe) ; and i think this is a default behavior way better than centering !
In fact, the user should call Window::SetPosition if he want to center the window, not SFML.