Usability problem in the widespread 'is a' approach

As introduced earlier, taxonomies as thesauri and classifications organize terms in a net similar to a tree. Hierarchically, the terms get ordered mainly by is a relationships, seldom by has a ones. Also there are relationships between terms that have "something" to do with each other, but cannot be ordered validly hierarchically. Like bird and bird cage. Some taxonomies force one parent term per node, i.e. a structure that easily can be identified as being mainly a tree, with exception of some cross reference like associative relationships. Other taxonomies don't force the one parent per term rule, so that such a one might easily look more like a net but like a tree. The prior ones are called "mono-hierarchical", while the latters' label is "poly-hierarchical".

What's the problem?

Taxonomies don't do anything more but relating the terms to other terms. The task to define the terms they leave to dictionaries.

This presupposition requires that the one who's using a taxonomy already is a kind of an expert in the field organized by the taxonomy. If you're looking for a term you don't know where to look up, you're lost.

Say, some part fell from your car, and since then it doesn't move anymore. You don't know the term for that part, and the very one is too heavy, so you cannot just take it to the garage. -- Because of its primary is a nature, a taxonomy is of no use here for you. Another chance is to go to the garage without the part and attempt to explain the nature of it to a worker there. (So far, my university information science teacher guided me.)

So, the problem of a taxonomy is that it doesn't support the most straight ahead approach to identify an item -- to select the most conspicuous properties of the item the taxonomy -- broader: knowledge storage, e.g. a garage worker's memory -- already knows about.<<

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Updates: 20070624: Tagged the posting. Updated the posting style (layout) to my current style, such as using blockquotes when appropriate, more precise word picks, better grammar.