Enhancement shaman could technically go with either depending on what they wanted. 1 str gave 2 attack power. 1 agi only gave 1 attack power but also gave crit.
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I always thought there was some imbalance cause agi dps and casters got crit from their primary stat while str dps didn't. I was hoping they'd even things out, and they did... by taking it away from agi and int.