This is a rather interesting skill, but in my opinion, it is less effective than the ones mentioned above. Knowing all the spells is good, but a mage still won't be able to use them all in battle, and they can learn the most necessary ones anyway. Perhaps it could be useful to find something suitable against any opponent. Overall, the main advantage is that it allows you to vary the spells you use, depending on the specific situation in the battle, but this skill pales in comparison to most other absolute skills.