I can't imagine that this comes up that often, but it is definitely helpful if it did. The idea would be that one of the other alerts doesn't tell the whole story by itself. For example, set 10181-1 Eiffel Tower. Part # 3633 Dark Bluish Grey Fence piece is not a rare color or a part that appears in 3 sets or less. However, it requires 254 of them which is more than what appears in every other set combined that this part is found in.
So, if a part requires a quantity for a set which is above the quantity required in other sets, it should have a "rare quantity" designation. I'm ok to limiting this to items which are no longer available from Lego themselves if that is possible.
It would sure save from mistakes from being made trying to build sets from scratch.