A lot of the time people are agreeing to lot on this or that job which they aren't using, when making pick up groups. And a lot of the times strangers just break the rules and steal the drops.

Wouldn't it be possible to bind our need/greed to a job of choice when entering the instance?

Say you go in, it asks what will you be lotting on, you go in as PLD but pick BLM, so you can only "need" on BLM gear.