I'm just unclear about the rationale for this change. To anyone looking at the permissions of members at the main group level, it's not possible to determine who has access to control the membership. A moderator of a subgroup gets access almost by proxy.
In our case, we planned to have a centrally maintained membership list with subgroup moderators being able to add previously vetted members to their own subgroups. The ability to have a centrally maintained list is one of the reasons we are looking at groups.io instead of a set of disparate Y! groups whose overall membership is difficult to control. This latest change blows a big hole in our plans for that.
If the ability of a moderator to invite members to join a subgroup remains then I think it should be optional (distinct from Direct Add) - in which case we would disable it.