custom url to username and maintain urls for controllers

[eluser]Unknown[/eluser]
^how is this approach different or better than just specifically defining the reserved controller routes like this:
$route['tv'] = "tv";
$route['radio'] = "radio";
$route['students'] = "students";
$route['media'] = "media";

is there anything wrong with my approach?
i am curious... as i want to implement it and want to know the best possible way.