When should you use class and movieclips?

Hi, I’ve read tutorials on it but I’m not really sure when to use class files for movieclips (specifically particles such as falling blood and debris). What’s the advantage of having several movieclips each with a linked class file (each with an ENTER_FRAME event) over an array which controls each particle with a single ENTER_FRAME event?

This is the tutorial in which movieclip has its own class file: http://www.kirupa.com/developer/flashcs3/movieclips_classes_AS3_pg1.htm