As far as the hair models PM goes, 8bitcrab just told me he didn’t realize how long it got, and will be closing it.
As for the private category vs PM difference, the other distinction is that one requires a group to be previously created by an admin (the category) where the other can be thrown together on the fly by simply listing usernames.
I know this is slightly off topic, but if the idea here is to get long discussions out of PMs, can you make it easier to create private categories by allowing moderators to create groups. In the case of the hair PM, that was 3 or so users discussing a mod, and didn’t need to be public. However, even with a moderator involved in the discussion, the only way to keep it private was with a PM…(without escalating the request to an admin)