What does it do? I've seen no mission that needs it.

What effect does it have/give?

Its just another racial trait on the Duty Officer like Telepathic or Logical. It appears on the 4 races that created the Federation. I have seen it listed as a success or critical success amplifier on some diplomacy / mediation doff assignments. I don't think I've seen any doff assignments that absolutely require it though.

its probably needed for certain diplomatic or trade missions.

Pretty common on diplomatic missions that I've seen.

OP: As far as I've seen so far, no doff trait does anything per-se, only their job title. The traits are just modifiers that make them better or worse at certain assignments. As such, ones that sound negative like unscrupulous or unruly aren't, there are plenty of assignments where they increase the success.