Take this scenario: Rectangle *r = new Rectangle(); r->draw(); // calls Rectangle::draw(GREEN) Shape *s = *r; s->draw(): // calls Rectangle::draw(RED)

Take this scenario: Rectangle *r = new Rectangle(); r->draw(); // calls Rectangle::draw(GREEN) Shape *s = *r; s->draw(): // calls Rectangle::draw(RED) Intuitively, you would not expect that a function changes its behavior when it's called from a pointer of a different type. I can easily see this leading to very a fun-to-reproduce-kind of bug.