An example of this would be brewfest. As a holy paladin, I find the healing trinket dropped formt eh Coren Direbrew encounter. Being a healer I needed on it only to find that the dps Shaman had needed also and won the roll before being able to contest he'd left the dungeon. As you can understand I was pretty angry at the blatent ninja and I suggest that blizzard have another look at the system.
They either A need to specialise it so only classes can need on items they can use mainspec. Or assign a roll bonus to stop this ninjaing occuring.
Another example also was my friend in the same dungeon needed on the tank trinket as a tank to have it ninjaed also. Whist this is one encounter its happened in others also and its disheartening to know that what should rightfully be mine because of my role goes to a player who doesn't equip it or disenchants it in front of your eyes. (Another thing which annoys me)
Now people may say I'm whining or complaining but I'm not. I'm merely trying to highlight a flaw in what is supposed to be an upgraded system. At least before when you could see who needed and greeded the ninjaing was less likely to occur as a result of players actually supporting the role it belonged to.
e.g. Old system warlock needed on healer trinket, other players said that healer had prio when both selected need.