Currently, the get_categories function takes *most* of the parameters of wp_list_categories, but it lacks a key one: depth. Sometimes, you need more flexibility than wp_list_categories allows, and get_categories works well for that, but it requires some unfortunate hacking to list only one level of subcategories.
I'm sure folks aren't exactly *clamoring* for this added functionality, but it would plug an odd hole in the code.