I dunno about dueling but it does work great on PKers at the lower levels when most are relying on big physical damage. In conjunction with a good bone armor they typically can't hurt you. After all a 200 hp hit is 100 wiht PvP reduction, then toss in another 33% and your only getting hit for 67, or basicaly 1 point bone armor, 4 points bone wall, or 7 points bone armor.
Truth is most pre level 18 (at least in HC, where saveing of items isn't as possible) don't hit that hard anyway, even with critical strike (which is factored in after the 33% redux).
Also is curse of choice for that matador build too where you want the damage reduction but not the slow down. Much like the IG mentioned above.