Agreed, I felt unpleasant writing it. But as a general utility class I don't like passing the reader to the method with each look-up because the reader is not used as a key in the cache. 
Also, I wanted in future to allow for looking up background frequencies in different ways e.g.

1) Using a subset of the index (perhaps using a parent agg as a backdrop - see https://github.com/elasticsearch/dev/issues/92#issuecomment-35797451 )
2) Examining the frequency of concepts in a set other than single indexed terms e.g. a numeric range.

So for now I was leaving this as a "TODO" area - maybe I can add a comment in the code to this effect?