There are situations where sales people who compete and are Standard ACT! users, need to not be able view other sales people's activities in the calendar. The facility of granting calendar access allows only for either View or View and Schedule.
Private activities and Limited Access do not provide for strict calendar security and making an activity Private hides it entirely except to the creator of the activity.
Another option might be to modify Private status so that an Administrator or perhaps also a Manager User has rights to see all users Private activities. Perhaps a user is able to knowingly grant that an Administrator and perhaps Manager User or named users can see Private activities.