I feel that, since you also earn a soulbound recipe for achievement apparel, the apparel earned from achievements shouldn't be soulbound. Some of these pieces will take a long time to earn, so surely we should get to sell/give them to other players?
To me, soulboud items are something that should be reserved for handouts, which will effect the economy more if they can be traded around than something someone earned over the course of days, weeks or months of gameplay. I would argue that players would feel more rewarded by currency if it's item that they don't like.