In reply to Sean Fenton.

It's about the end-user not having the flexibility of some of the most basic CSS needed for truly designing nav menus. Because the end-user can change some values such as the text and background colors, those colors may clash with the theme's other colors for borders and link backgrounds, for example. They also do not have control over :hover and :focus states of those colors. Thus, using CSS is the better solution at the moment.