I agree that a lot of other documents don't have the metadata that they need to allow them to be managed in this flexible manner. But that doesn't mean that a fixed directory structure is a fundamentally better approach - it simply means that these file types (or the users that create them) haven't caught up with the benefits that proper metadata tagging would give.
In answer to your question about Project X - if the documents were able to be (and actually were) tagged with "Project X", and "2009", it would be extremely straightforward.
Using a fixed directory structure, you're relying on the fact that you know where it was stored. Did you choose to store your projects by date? If so, can you remember when it was done? Was it stored by client? Can you remember which client it was for? Was it stored by development team? Which team created it? In a metadata world, all of those things could be tagged independently against any relevant document and you simply do a search for the attributes that you're interested in.
In answer to your question about Project X - if the documents were able to be (and actually were) tagged with "Project X", and "2009", it would be extremely straightforward.
Using a fixed directory structure, you're relying on the fact that you know where it was stored. Did you choose to store your projects by date? If so, can you remember when it was done? Was it stored by client? Can you remember which client it was for? Was it stored by development team? Which team created it? In a metadata world, all of those things could be tagged independently against any relevant document and you simply do a search for the attributes that you're interested in.