Yes, it's a real, real tight niche right now. Much less outside interest than one might infer from hanging out at the forum all day
I'm not entirely sure what you mean -- keyword, keyphrase, these are largely interchangeable and depend on who's doing the talking, eg. Google AdWords simply refers to all sets of terms as "keywords" in their ad management, regardless of whether it's technically a word or phrase.
Low volume terms also have value though -- if you look at the ratio of searches to results, you'll sometimes find some real gems to target (ie. a low ratio of results to search volume generally makes a keyword easier to rank in the search results, so they are not without value when you're trying to increase your site's visibility to web searchers.)
Something that should be kept in mind when using any kind of keyword tool is lag -- none are updated in real time, so your trends will always be slightly off. I would imagine that coming back in three months and running another search for Bitcoin-related terms would produce many more results, and coming back a year after that would produce even more, as the topic gets covered more and enters a more broad spectrum of discussion and inquiry.
Frank