I may have overstated the claim in that sentence. More accurately, I think taxonomies aren't, themselves, useful. If they don't help you solve problems, you're just stamp collecting. I've seen that happen way too many times - otherwise smart people thinking they're being productive by writing a whooole lot of useless classes, where a few functions and some if statements would do it.

But don't get me wrong - I'm totally in favour of having common interfaces. They aren't great because they form a taxonomy. They're great because it helps us abstract. Whether its Iterator or different software implementing HTTP, interfaces actually help us solve actual problems.

> Knowing that AES and Twofish are block ciphers is useful when dealing with cryptography.

The useful fact in this example is that they both do a similar thing. They form a type class from their common behaviour. Behaviour they may share with - for example - a compression algorithm. Thats what actually matters here.

I think this is a lumpers/ splitters thing. The problem with a tree structure is that it assumes a very particular shape and you have to distort your understanding of the world to make that fit reality which has no such inherent pattern.

So, the taxonomy isn't useless, but, it's also never sufficient.