It's true you get .04% block/dodge/parry per defense, but you also get .04% for a mob to miss you when you attack.
15 defense rating * .423 defense per rating (at 70) = 6.3 defense
* .16% (block/dodge/parry/miss) = 1%
Basically you get .75% avoidance from this enchant and .25% block.
However, you only need 490 defense (330.4 rating from your gear) at level 70 to be immune to criticals from a mob three levels higher than you. There are few exceptions to this rule, such as Malicious Instructors in Shadow Labs and Assassins in various instances. It's pointless to continue stacking defense after you reach the 490 benchmark because other ratings increase far more rapidly than defense (15 dodge rating is .79% avoidance and 15 block rating is 1.9% block.)
So in short, only get this enchant if you have not reached 490 defense. If you have, go for 150 HP or +6 stats because HP is your new best friend.
15 defense rating * .423 defense per rating (at 70) = 6.3 defense
* .16% (block/dodge/parry/miss) = 1%
Basically you get .75% avoidance from this enchant and .25% block.
However, you only need 490 defense (330.4 rating from your gear) at level 70 to be immune to criticals from a mob three levels higher than you. There are few exceptions to this rule, such as Malicious Instructors in Shadow Labs and Assassins in various instances. It's pointless to continue stacking defense after you reach the 490 benchmark because other ratings increase far more rapidly than defense (15 dodge rating is .79% avoidance and 15 block rating is 1.9% block.)
So in short, only get this enchant if you have not reached 490 defense. If you have, go for 150 HP or +6 stats because HP is your new best friend.