It has an estimated proc chance of 5% to silence the enemy for 6 seconds - because of that, it is a somewhat handy weapons for Rogues who often have the task of silencing casters. The high speed of the weapon might also make it useful for fury Warriors.
Hotfix (2009-04-08): "We have updated the Silent Fang so the chance to proc decreases starting at level 61 and is no longer possible on targets level 71+. The proc effect also causes diminishing returns with controllable silences such as Improved Counterspell and Strangulate. [1]"