The highlighters are great because they help you figure out why your query matched a document. Unfortunately when they are used in conjuncture with any kind of analyzer that stems the words in the query, it is difficult to figure out WHY the query matches a document. The FVH highlighter and the pre/post tags help a bit, but it is not clear from the docs what the expected behavior of the FVH highlighter is.
I wrote a post a while ago about about the topic but got no discussion going. So I've written a longer deep dive about the subject here.