The simple truth is that the need/greed system has been around for ages. It doesn't matter what system you implement, people will abuse it. People have been complaining about it for ages. Most veteran MMO players just ignore loot ninjas and move on. The hope is that they will eventually not be able to find groups.
I don't know if there are guild banks yet(not being in one myself
), but just wait until the ninjas start emptying the guild vaults.