Well, technically, the feature is called Quick File Search not Quick Folder Search . Therefore I would imagine most people are using this to search for files rather than folders, thus lessening the value of cmd+down.
I guess I should move this to feature suggestions as it would allow quick navigation to deeply nested folders where you know a file name but not the folder name or where the first result is a file in the desired folder.
edit: to correct my technicality to make it technically correct.