You said [about Ouija boards], "Whether or not it falls under the prohibitions of the Law of Moses against witchcraft, sorcery, or divination, the use of this device and others like it doesnít give us anything we need to know." (What Does the Bible Say About..Ouija Boards?) If the Ouija board does fall under the Law of Moses against witchcraft (which I think it does), then why would God warn us not to practice witchcraft if we can't contact spirits?
As I see it, it makes no difference whether we can truly contact spirits or not for God to prohibit the practice of witchcraft. Even if the practice is useless, just to rely on it or believe in it is to deny Godís power. The problem with witchcraft as described in the Bible is that it puts spirits, or tea leaves, or animal entrails, or drugs, or whatever in Godís place.
In that sense, witchcraft is the same as idolatry. Yet God does not acknowledge that idols have any power. See Isaiah 44:9-20 for a description of the vanity of making idols. Still, God has commanded against idolatry, even though the idols have no power. In the same way he forbids consulting spirits and horoscopes, even though doing so is really worthless.