You think this works like the Prisoner's Dilemma
Yes, in one interaction, people can abuse the need / greed system and win. This is a game where one character will typically be played for a long time, though, and there are social consequences to ninja behaviour above and beyond
pure loot consequences. For example, being unable to group because you are in ignore lists, being booted from the flashpoint or operation before the end, valuable players leaving the group because of your behaviour leaving you unable to finish the content.
In practice, the current need / greed situation works pretty well. Everyone knows the rules, most of the time people behave decently. Your proposal adds nothing.