I would say it’s perfectly fine to have pet causes, when the favoritism doesn’t abuse public authority or a position of trust, for much the same reason as your example with loved ones.
Every child deserves to think that his parents love him more than other children, who hopefully have their own parents, or else need special care and attention provided.
Similarly I think we should accept that everybody deserves to think that they are part of a network of overlapping affinity groups, and that if a small fire started in our yard while we weren’t home, our neighbor would stop what he was doing and put the fire out.
One thing you might want to add about the potential costs of any conflict is that it deters long term investment. Nobody plants an orchard that will only give fruit in 15 years if they think they might have to run for their life in three.