User blog comment:Zilla4444/AD upgrade for Chalice of Harmony/@comment-5045559-20120610183142/@comment-5045559-20120610194605

MBR and Wit's are both tanky dps items, they're for shyvanas and the likes. MBR on AD carries tend to fail, because of the huge cost and lack of Mpen, while their targets (high HP tanks) will usually have balanced, high amounts of both resists. As far as i know decent AD carries get LW instead for that reason.