This talent was removed from the affliction talent tree in the 2.0.1 patch. It used to provide a 5% extra slow for Curse of Exhaustion (which used to be base 10%). With the advent of the patch, however, this talent was basically added into Curse of Exhaustion, raising its base to 30%.
Really, if you're going to invest in Curse of Exhaustion, you had better plan to spend another four points here. CoEx is pretty much a waste without it.
4 points in this talent reduces the target to 70% of its normal movement speed. Once every 3 minutes you can also add Amplify Curse, making it 50%. CoEx can actually be very useful in catching those enemy players who are trying to escape from you and/or your allies, especially if they're carrying the flag in a capture-the-flag game.
In PvE, Curse of Recklessness will already prevent targets from running away from you or bring them back if they do, plus it lasts for 2 minutes vs 12 seconds for CoEx.