Skills use a multiplier of the atk/matk (or in wonderer's case <i know not on topic of aco but still vaild> men mixed with either atk or matk) to apply its damage vaule before def/mdef is calculated , using an example of say 350 atk. You get like 40 atk from the passive that accounts for a 10.25% increase in overall attack. Id say thats a pretty nice increase from a "passive" ability. Thats enough to offset a fair (ill stress that a bit ) amount of def values. No matter how yea look at it, it is very useful even with weapons that offer 400 atk. Granted that % gained appears to be less as you start breaking 600~800 atk/matk vaules but darn if it is still a nice for a passive.