By joining the Anama, you get a bonus to priest spells, magical efficiency and magery; assuming you have a point in them before hand.
The side effect is that you won't be able to cast mage spells.
I have a party with 2 priests and 2 mages. I killed the 2 mages before joining the anama. In effect, the 2 mages didn't join the anama, while the other 2 priests did. So my priests got all the bonuses, while my mages didn't get any penalties. However, if you had joined the anama with all 4 of your party alive, then that would be a different story.