Jackrabbit and Exo provide a similar feature:

• http://wiki.apache.org/jackrabbit/ExcerptProvider
• http://wiki.exoplatform.org/xwiki/bin/view/JCR/Search%20Result%20Highlighting

While this isn't mandated by the JCR spec, it seems like a fairly common need, and a nice-to-have feature, so users of the repo don't have to fiddle around the search results to hack something themselves (which is most likely going to be underperforming and..ugly)