The "group mode" defined in the course settings has two independent effects:
1. to set the default group mode for newly-created activities
2. to enable the groups drop-down in the participants list and in the gradebook
There's no reason why a user who wants 1 should necessarily want 2, or vice versa. These should be independent settings - in fact I do not see there is a need for a setting in the the second case; if there are groups defined, the drop-down should appear automatically.
One effect of this, that can appear like a bug to users, is that fact that in a forum using group mode the links in the 'Groups' column can be misleading. One might expect that they would take you to a list of group members, but in fact, if the course-level group mode is "No groups", then the links lead to a list of all participants.
I've commented on this same issue in the context of the gradebook here: http://tracker.moodle.org/browse/MDL-14255