Gregory Maxwell wrote:
Because adding the parents produces non-sense results because "categorization" is a flawed concept except at the most fuzzy and course levels: Reality doesn't fit into neat nested boxes (not even the N-dimensional ones created by multiple parentage). The two primary problems are semantic drift (the further away you get from a relationship the more not-quite-matching error accumulates), and multiple link types (we use categories to describe different types of membership, and while within a type the membership relation is commutative among types it is usually not). So with parentages you get chains like [periodic table]->[hydrogen]->[hydrogen compounds]->[water]->[places with water]->[beaches]->[beaches in america]->[beaches of lalaville]->[lalavill beach]->[Image:Ironmeteor_at_lalavill_beach.jpg]
Is an iron meteor a "beach in america" or a "hydrogen compound"? No.
True, but there are _some_ relationships that should always hold. All dogs are animals. All integers are numbers. All places in New York are in the United States. Arguably, any page which is in [[Category:Dogs]] but not in [[Category:Animals]] is a failure of atomic categorization.
Of course, there are also many relationships that _don't_ hold so strictly. Most dogs are pets, but not all. Most places in the United States are in North America, but not all. So, yes, some of the consistency checking will have to be done at least partly manually.
But really, I wouldn't worry about this too much. Sure, having a way to enforce some category relationships would be useful, as would automatically recommending others. But even if we don't implement it immediately in the software, someone will write a bot (or several) to help with it. It won't be perfect, but I wouldn't expect it to be much more broken than the current interlanguage link system, which we consider useful enough to keep deployed despite its numerous failings.
(While thinking about this, I thought back to an earlier discussion on this list (or possibly wikien-l, can't remember now) about the fact that there are essentially two types of categories: thematic and taxonomic. For the former, the "tag" model of atomic categorization is quite natural, but the latter would fit much more naturally into a strictly hierarchical model. It might not be an entirely unreasonable idea to formally split the two, perhaps even into separate namespaces, and apply different technical approaches to handling them.)