I'm afraid you are 100% wrong. The argument that rarity should equal power is a fundamentally broken one. Rare items will be chased and with the random nature of loot will be generated and will exist and over time, will become common amongst the used items.
If 1 in 1000 items generated is WoP then you sell the other 999 to vendors and use the 1. And then repeat the process. And so do the other million players.
What THEN happens is that the rare/powerful item becomes a required item to be accepted into groups/raids. Take a look at the Shroud - ever single shroud run on Orion stipulates that BossBeaters are required for melees.
Let me give you an older example from another game, Everquest. There was an item that could be used as often as you liked with no cooldown that gave you back spell points. Every cleric (and other spellcaster) wanted it, and it became required that a cleric had to have it to get into groups. So much so that the item was removed from the game, and the world changed so that it only worked in a few places. That is what happens when rarity = power.