That is the point.. if the tank needs the item, there is no way a non-tank can win it, but it still allows the fury warrior (who can roll need) to always beat a mage who can only roll greed in the event that neither tank rolls need. In the past, the fury warrior had two options.... roll need against the tank (and risk getting kicked) or else roll greed and have to roll against people who are just going to vendor it. This system creates that layer in the middle that most players wanted.
It will work out exactly the same as having a ´need for offspec´ button, but was probably a lot easier to implement since it required no UI work or really much effort at all.
There are still going to be problems.. bear tanks, DK tank weapons to name a few... but they are limited items and their aren´t that many specs that would roll need against them.
Good solution Blizzard