How uncertainty about fair use hurts (Selma edition)

Vox's Timothy B. Lee on why the movie Selma doesn't use the actual words from MLK's speeches:  "Most likely, the problem is that studios are extremely conservative about exercising fair use rights. And they have good reason to be wary.  If Paramount had distributed a version of Selma that relied on fair use to justify using some King clips, and a court later ruled these uses were not actually fair, it would have been a financial disaster. Paramount could have been on the hook for big damages and could even have been forced to cancel showings of the movie and destroy its inventory of Selma DVDs. ... So rather than take that kind of gamble, studios almost always insist that works be licensed, even if a plausible fair use argument exists."