Single Stores and Multi Stores: Extra category

One important Magento feature is to sell products in Single Store and Multi Store. In Multi Store one product could be sold in several stores.

Now, primary category field in product form is the field used to define which shop the product will be. But the primary category is a many2one field, so the product can only be sold in one shop.

The relation of between product and stores must be defined by the new many2many extra categories field (in Extra category tab) and not in "primary category" field.

For example a typical category tree:
Root Catalog/Default Category/Category 1
Root Catalog/Default Category 2/Category 1

Default Category is the root category for shop 1.
Default Category 2 is the root category for shop 2.

A product associated with these two categories must be sold in these two shops/stores.