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.
At the same time, this only works if more “disinterested” donors recognize groups that are for any reason isolated from such networks, or where the entire network received a correlated negative shock, so that “impersonal” aid funds are directed to improve their situation (because after all, that’s where the highest returns are).
I think this is a more robust system than having all aid be impersonal, because after all, what if the cost benefit calculation is wrong? So MacAskill can of course donate to whomever he wishes, including not donating to those he wishes to give to give the most, but personally I find that an example I will admire through observation rather than emulation.