Did you not read that section at all? If you lose all knowledge of powerful spellcasting, a) you lose your ability to continue to be immortal after this iteration, b) you lose your ability to defend yourself against enemies who haven't lost their ability to cast interdicted spells. The second one is really important when the process for immortality is one that inherently makes a lot of enemies! He specifically mentioned that dark wizards that tried use that technique to come back were easily defeated afterward.