Arradwen
Legend
Lets assume for a moment your average backstab was for around 4k, (which is a massive lowball since against cloth and leather classes mine range from 5-10k, highest being 11k on a mage) the 6% increased damage is giving you an extra 240 per hit on a 4k backstab and 660 on one for 11k, thats a lot more damage than 10 flat agi could give you. The reason this set works so well is during the opener if done right, with procs your weapon damage jumps up to around 1k, so with your huge backstab crits and SnD up your white damage is also deadly, it just has amazing output.