The average light jacket or windbreaker doesn't seem like much protection against a knife attack, especially considering how thin the fabric is, but most jackets really do offer decent protection against knife slashes.
Now I'm not going to claim this is firsthand knowledge but I've seen enough self defense videos to know a jacket can come in handy in many ways during a knife fight, and this video by Skallagrim just further proves the point.
He slashes away at his jacket-clad dummy and totally shreds it, but underneath the jacket the skin remains virtually unscathed.
http://www.youtube.com/watch?v=O5p6sy591Wg