The thing is, "now the opponent doesn't have an item and you do damage," is what Knock Off does, but it does so without the need of Magician, your item slot, or an extra turn.
Magician:
Magician sucks because it relies on the target's item, and therefore, you have little control over it. (Thanks to HeaLnDeaL for that quote) This is especially true for this concept where the item must be consumable, which means the CAP is, once again, useless unless the opponent has a consumable item.