I frequently use the "show subtasks > as separate tasks" when filtering tasks. If I don't, I won't see subtasks that match the filter if the parent doesn't also match. The main problem (unless I'm doing something wrong), is that its subtasks aren't shown/grouped under their parent... which I understand conforms to the "as separate tasks" setting, but it would be great to be able to group by the parent, even if the parent doesn't match the filter results.