I see your point.

However we are taking advantage of the feature where the groups are linked to courses and users enrolled in a particular course are added to the relevant group.  The courses that we are running that use the groups are for 6 months. So the discussions, forums and sharing of information etc is specific to the course that it is linked to. When the course is complete the activity / information in the community is no longer current / relevant.

We set up a new community when a new course starts so we want the old one to close. Otherwise we are will end up with multiple exam prep groups within the community.