First, see if the school is receptive to them. Some schools are adamant about not accepting updates of any kind except for things like institutional actions. That being said, I'm not entirely sure why they are so widely vilified on SDN. Two schools that I interviewed with encouraged us to send them updates or letters of intent if we were sure we wanted to go there. I doubt they're particularly efficacious, but if they accept them I see no reason not to send them a letter that emphasizes your fit with the school and that does not sound entitled or desperate. Obviously, if they don't accept them then don't send the letter because you will come off as annoying and unable to follow rules.