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Once you equip specific job stone, lets say BLM one. You get int and other stat boosted. Whm = mind and so on. So, If you take that stone away, it's like downgrading your character. Flare is powerful spell and important rotation tool. Manawall is emergency instant defense. And even freeze and convert has their use. I also don't think many people would appreciate you when you join high lvl raids as a thaum. I'm just saying :) interesting idea tho.
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You can have raging strikes without changing to Thaum.