I agree with the others. I let my parents know who is leaving and when as we usually have a little going away party on their last day. I do it so the parents aren't wondering why they left, the kids have time to say goodbye to their friend and the parents can refer me to anybody they know looking for a dcp. My group is small (often only 4-5 families) and the parents run into each other here and become acquaintances and a few of them know each other outside of dc so it's a close group. In fact, one of my current dck's is having a bday party this wknd and one of my past children will likely be there.
I had 3 leave at the same time last year (end of June) and than 2 leave this year within 2 weeks of each other. None of them leaving had anything to do with me but I felt that it looked bad and if I didn't have the policy I do, I can imagine the other parents might be wondering why they all left together! lol. It was all things like going to a dc centre, parents loosing jobs, kids moving on to preschool so it was nothing overly private and the parents leaving usually tell the other parents anyways so it's not often I have to say anything anyways, just the odd time to a parent or two.