That is the same reason why having a third magic slot doesn't increase your stats! We decided that since these slots are so rare, we don't want any additional points for special prizes, especially since you cannot quest for them.
That is something that will not change in order to be fair. Also, having two companions or a third magic is not always a powerful addition since you have two competing voices in your head and now a third magic to try to control! (This is the IC justification).