The factory-installed rack poses some peculiar challenges. Sometimes they provide a tremendous basis for the addition of towers and bars, creating a smooth, integrated system. Sometimes they're a little wimpy. Most of the time, they're just fine and dandy.
The best news about using the existing factory rack base is the ease of installation. Easy on! The thing to remember is that it's easy off too, so don't forget security. Yakima factory racks install with minimal or no tools, so they're user-friendly for user installs.