Faceless (or mono-faced) supporting NPC extras have feelings too, so it pays to be nice to them.

Besides, you never know when that beggar in the street will turn out to be an evil enchantress on the prowl for victims to slap curses on, a polymorphed dragon looking for someone to eat, or a vampire. If your GM is doing a half-decent job, some of them will be.

And if you're running a game, make sure some of the anonymous NPCs in your world are indeed things like this. Well-motivated fear is a much better motivator for good behaviour than potential good karma.

Transcript

Anakin: Good luck capturing General Grievous.
Obi-Wan: We need to discuss some serious issues, Anakin.
Anakin: Of course. When you get back.
Obi-Wan: Yes. Meanwhile... don't do anything I wouldn't do.
Anakin: Trust me.
{Obi-Wan turns to board a troop transport ship.}
{Anakin watches him go, with a cunning smile on his face.}
GM: You meet the clone troops on board their transport.
Obi-Wan: Okay, when we get to Utapau, I'll go in to scout the situation.
Clone Trooper: Yes sir! Although, we could go in first to see if it's a trap.
Obi-Wan: That's... very nice of you. But don't worry. I'm sure it's a trap.
Clone Trooper: We could go in first and trigger the trap.
Obi-Wan: You guys need a union.