How to have parent taxonomy terms include their children terms' nodes? I have a hierarchical taxonomy, and I'm currently using the Taxonomy Menu module to display navigation links. E.g.

+ Dairy
  * Milk
  * Cream
+ Fruit n Veges
  + Citrus
    * Lemon
    * Lime
  * Pumpkin
  * Tomato


Each of the links refers to a taxonomy page, which is nice, but the page for "Dairy" only includes nodes that have the term "Dairy". It doesn't include "Milk" and "Cream" nodes, for example, which is what I want.

What's the best way to implement this? And "what's the point" of sub-terms, if they don't seem to have any semantic meaning?
Install the module http://drupal.org/project/hierarchical_select and check its configuration and you will find an option to "Save term lineage" select it and all the level of terms will be saved with the node.