I need to create a member group containing members from other member groups. Searching for each user by name takes a long time, and I have enough users that finding them in the list takes a while. If I could filter the 'unassigned members' section by member group, it would cut down the list to make this operation faster.

The only scenario I can think of where I wouldn't potentially want to filter a user list by member group is the 'Assigned Members' section on the edit screen for that member group.