[narnianal]

I would very simply summarize it like this: If you know which data you look for and say "give me this piece" then you want to use a hash table. If you don't know what you are looking for and need to filter/search, then you use a b-tree. So actually you want to use both structures regularly.

Not sure how that general principle really applies or if it's just a quirck my brain came up with, though. If you look at distributed hash tables for instance, they are hash tables on a bus network in some sense, but they are used for search/filter tasks over multiple nodes.