Consider text summarization first: many well-known models such as MMR (Carbonell and Goldstein, 1998) and MEAD (Radev et al., 2004) rely on the reoccurrence statistics of words.
if we switch any word w 1 with another word w 2 across an entire corpus, the ranking of extracts (often sentences) will be unaffected, because no wordspecific knowledge is involved.