In the rough and tumble of swordplay, it really is the quick and the dead. Hunters are taught how to parry from early on in their training, but it does not come naturally to all of them. Some are just a lot faster than others, and always seem to have their weapon in exactly the wrong place, at the right time...
Sometimes the extra strike deflected can prevent the killing blow.
Deflection is a Hunter (second tier Survival tree) talent. The talent grant a 1% increase to parry chance per point spent.
For non-tanking classes, usefulness of this talent can be highly varied, depending on spec and preferred play style. This talent is useful for any who plan to PvP, in addition to:
Hunters who are willing to 'off-tank' temporarily while a trap cooldown runs out, or who want to be able to handle a mob a little longer in a group when pulling it off a caster until the tank or the Hunter's pet can pick it up. This talent is often taken in combination with Deterrence and Counterattack.
Patch 1.7.0 (13-Sep-2005): Will now increase parry chance when using Fist Weapons.