The note feature (and its interchangeability with memoQ, BTW) is a real time-saver for me.
When marking a part of the target segment, I only need to do 2 key strokes to have a note ("New note" and "Confirm"). But the same with the source segment does not work. I cannot mark any part of the source segment, type the "New note" shortcut and confirm.
The need for this might depend on your habits, but I do not think this wish is too strange.