As a marketing consultant I have been asked many times to review themes for clients. While I think that it is important for maintaining the brand image of a theme to have customized products for the demonstration, there should also be a requirement that the demo products which can be downloaded at the Magento Commerce website be included in their entirety in any demonstration shop.
The functioning of a Magento storefront is more than just how you display a product image or layout of a category. It’s how you handle validation errors on products and display tiered pricing, upsells, product attributes and reviews. If there was a requirement for the inclusion of the stock demo products and categories, all of these aspects would be easily answered and provide additional sales to developers who have accounted for these aspects in their development.
There would be some awkward licensing issues with that and surely the way in which you rightfully said you need to structure the products should depend on the bespoke products of the brand in question-if it is a clothes store then the structure will be different from that of football equipment shop and so on.