If you run as a dragoon and a dragoon piece drops, you can roll need for any reason at all. If you want it for your collection, that's good enough. Need means you get first dibs, so if you want it, go for it. Greed means you got dibs if no one else wants it. The way gear works, there will be times when you want something but won't be able to need on it. Take your rewards when the game gives them to you, and be gracious about not getting loot when it's someone else's turn.

OP, it was very generous of you to let the tank have have the spear, when clearly the tank didn't need the weapon either. I think you should have rolled need, though. In fact it was rude of the tank to ask you to give up your reward. Do they also give up their rewards when they have a chance to roll need?

There's not very much gear that people "need" in this game. There are lots of ways to earn good gear, so don't be guilt tripped into giving away rewards to selfish people.