Nearly all items with Dispersion were
supposed to be flagged for cloth users only, but a couple slipped through the cracks. A couple of items (like the Dreadscale Greaves) were intended for priests and crusaders, and were actually given a different version of the proc, which had a lower rate on it.
As far as an item being intended for cloth and equippable by non cloth, that is where the two "ancient" armor versions come in. Disallowing players that have already spent time and money acquiring an item from wearing it is not a good solution. So instead I reduced the effectiveness of the proc enough that the item can still be worn, but is no longer overpowering. Any new pieces that drop have the classes that may use them properly set.
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