User blog comment:Mnzombie/Issei's Peerage/@comment-222.222.236.174-20140110043127/@comment-1359322-20140111040150

......Evil Pieces are meant to be used in a way where it enhances the servant's ability. Take Kuroka for example, she's a spell-type and you put a Rook Piece into her, even if you do that, you would waste her talents cause she can't fight in close combat which is the Rook's trait. Same goes the other way, you let a Power-type use a Bishop Piece when he/she doesn't have magic talent, then it would be like mismatching their traits.