Just want to chime in here for a bit. Needing on a boe that is not relevant to your class/spec, proceeding to sell it for gold and buying the piece you needed is a totally acceptable way of using 'need' in my personal opinion. The fact that the item has a certain value makes it valuable to EVERYONE. Therefore, paying 300g (example) for a piece of equipment should be viewed as exactly the same as obtaining the item from a need roll. It is a very logical train of thought.
Now ofcourse, from the goodness of your heart or whenever you feel generous you may decide to 'let' the other guy take the item because it so happens it is a bis item for his current class/spec. But if you think it through that just means you are giving him 300g (example). At the same time you basically 'passed' on your own bis item.
It is NICE if you pass it to someone that has the spec and class for the item that just dropped, it would be a gift to said person, but in no way should that person feel entitled to get the item. He should realize he was just gifted a nice sum of gold.
This is such a logical train of thought, it is strange to see how people fail to see this reasoning.
Ofcourse, needing on a boe in a guildgroup or a group where there have been set rules regarding to loot and who gets it in advance, that would be considered ninjaing. As well as needing on soulbound items that aren't relevant to your class/spec, again, ninja, because those items hold no large value to the person that ninjaed it other than vendor gold.