They are often taken with knockback abilities and items for greater effect. Their unique property works even if knockback has been inflicted by a follower or another player. Damage amplification is multiplicative to other similar effects. However, a complete immunity (not resistance) to Knockback also negates the damage bonus.
Prior to the pre-expansion patch, the Strongarm Bracers were the best tanking bracers, as they had a guaranteed +8-9% Life roll.