Well lets look at classic rogue and ninja jobs and how they work, we don't even have to look just inside FF. They both use low damage and high critical hit rates/damage to do their dps; they use str as their main stat but dex can often be an equally good stat sometimes on par with str. looking at it this way it would 100% make perfect since for Rogue/ninja to wear str gear and get dex traits to supplement it's crit damage instead of it's base damage.