We always played that you do get to keep the defeated card as experience points. It was a "legitimate" combat, that you add the Str/Crft of the enslaved card using the Staff shouldn't be any different than using any other weapon or follower to add to your die roll. Of course, in the original version of the game, you didn't amass Craft experience points by defeating spirit-type monsters, you could only trade-in Str (7:1) to bump up your Str; you had to acquire additional Craft by the various other methods. So if you used the Staff to help defeant a Strength-type monster you kept it for experience to trade-in for Strength when you had at least 7 points of that from defeated strength-type monsters; if you used it to help defeat a spirit-type monster...you survived the battle, lucky you.