There seem to be 3 benefits of using this kind of summoning:
- You don't need to beat the spirit's successes with a Conjuration + MAG roll. Just need to resist enough drain to stay conscious.
- You can command the spirit to execute the service as a free action instead of a simple action.
- The drain is limited always stun and it's limited to 12.
If you conjure a force 8 spirit using normal conjuration and have MAG 7, you may end up with 16 physical drain, if the spirit gets and extremely lucky roll.
Forbidden Arcana, p. 92
When a mage summons a spirit using the Tarot, normal Summoning rules do not apply.
Instead, the mage declares they are Tarot Summoning, and then shuffles the deck and draws a card (a Complex Action). The mage must resist Stun Drain equal to the Force of the spirit indicated by the card (use Force 10 for Major Arcana). If
the mage remains conscious, the corresponding spirit is summoned for one task only, and it may be commanded by the mage immediately with a Free Action. If the requirement for the card has not been met, no spirit is summoned.
Additionally not being able to combine the Arcana mentor spirit with the Tarot tradition doesn't seem to be the intended rules. The German translation features a new sentence, even though it does nothing about the "first initiation" phrasing and isn't expressed well.
Das heißt auch, dass ein Anhänger der Tarot-Tradition zuerst initiieren und die Metamagie Weissagung erlernen muss, bevor er den Schutzgeist Arkana wählen
kann (und umgekehrt).
Translation
This also means that a follower of the Tarot tradition must initiate and learn the Divination metamagic first before being allowed to gain the Arcana mentor spirit (and the other way round).
This indicates that the actual restrictions on choosing the mentor spirit regarding metamagics are supposed to be:
- You need to be able to choose the next metamagic freely
- The next time you reach a new initiate grade, you must choose the centering
or adept centering metamagic.
The drawback is of course that the mentor sprit costs 10 Karma, since you need to buy the positive quality after character creation.