What is the reason behind the current menu structuring I'm not much of a Drupal Ninja; from what I saw at http://drupal.org/node/788972,
Drupal seems to have a menu structure like Joomla, in the sense that it allows menu items to be created under menu categories.
Why are we using this unlike a central menu items section, from which we can link an item to multiple menus, if needed?

This is what I can see in Wordpress. In the latter method, I can see reduced redundancy and a single point to change multiple occurrences of the same link. 

I would like to understand the logic/reasoning behind the Drupal menu structuring, not understanding how Drupal compares to Joomla or WordPress.
You can create as many menu items as you want and have them point wherever you want. You can do this in code and in the UI.

Drupal menus have plenty of flexibility, especially with contrib.

More:

You can set up many different menus (and not just one overall hierarchy) because that way you can have more than one menu. You can put them wherever you want on the page. You can have a sidebar menu and a top-strip menu and whatever else.

Internally, the menu system is really a routing system. Drupal uses the same set of code for routing requests as it does for presenting menus in web pages. Each route/item can have its own permissions, so the list of items the user sees is determined by their role, automatically.

In short, to recap: The reasoning behind Drupal menu structuring is that it's flexible.