Why, Where, and When to use reference pointers in filters/hooks?

Question

Why, Where, and When to use reference pointers in filters/hooks? What are the potential cons of not using them when suggested or required? Just looking for a more detailed answer than the codex provides and maybe some real world applications of this.

For example: add_filter('some_wp_filter', array(&$this, 'my_function_to_filter');