conditionally-require

What are the Advantages and Disadvantages of conditionally including a class file with in a method of another class in PHP?

In this question about including all the classes in a directory for an interpreter that needs all of them, it was suggested that a better way to handle the problem would be to conditionally include only the Command file that is needed for this iteration of the interpreter. For example, instead of this: require_once('CommandA.php'); req...