I did a search and was looking into the purpose of sub topics as it stands in the current version. Mainly what I have found is that if you complete a ticket using a sub topic but then later do a search for the Parent Topic you don't result with the item that you completed as a sub topic only the items specifically filed with the parent topic. Isn't part of the reason of having Parent/Children heirarchy the fact that if something is a child it should also be located when you are looking for the parent category especially for statistical purposes.
Also I know there is a mod for automated drop downs so you pick a parent, then choose the next level and so on until the user gets to the topic they want but will this ever be built into the main application or will it always be in MOD form...