There is a content profile module that basicly uses standard nodes for user profile data. When will you want to use it? When normal profile resources run out. This is usually when you want to have a more complex fields for user or have some backreferences. As basicly you will use then nodes and nodes have much more other modules that bring some extra possibilites. One thing that could be useful to you if you already built your profile is profile migrate which will migrate your profle fields to CCK node fields(content profile fields).