Racial prejudice encourages large-scale cooperation. Racially prejudiced individuals will happily sacrifice their personal well-being for the good of the (racial) group, even if the group is much larger than the Dunbar number. While this can certainly be useful under ordinary conditions of nonviolent competition, it becomes vital when the group faces an existential threat as a group.

A rogue paperclipper in a mostly Friendly world can probably only be stopped by racial prejudice--to a rational creature, it's always easier to feed him your neighbor than it is to fight him.