if i understand it correctly, being closed does not mean being private, all post will always be visible to anyone wanting to read and will show up in New Posts (please correct me if i'm wrong on this) So being a closed group will not get you privacy and i think that is a good thing, MDC is about sharing and learning from others imho
what a closed group will mean is that folks have to be approved before joining and all group have to be joined before they can get posted in.
i think MDC is dong a pretty good job of clearing out spam and trolls, it would be a shame if we feel we have to build roadblocks for so many folks wanting to be a part of out community, to deal with the few that are not welcome. let's just all work to flag and remove those that are destructive and keep the gates as open as possible for the others.
I for one follow a handful of folks i know that ended up in different DDCs than me and it is my joy and bliss to finally get to congratulate them when they birth, that is just one of the many reasons for those that are not "IN" a DDC to be posting in one.