It was originally a proc, and it was changed to charges about the same time Mistwalker was nerfed. I helped three different Ranger friends get the bow way back pre-Kunark, and all three were disappointed when they got it.
"Oh! It uses charges. Everyone told me it was a proc.
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I remember thinking how much it sucked that the ultimate Ranger weapon was really just a fleeting chance to use some high damage spells. My friends saved the charges for fights against Vox.