I don't think it can ever be used as it always introduces reasonable doubt. It is important for convictions to be safe or guilty people can go free on a technicality or innocent people convicted. If the amount of DNA at the scene is so small that it could have come from transfer rather than the person's actual presence, it introduces doubt.
Obviously if there is a large DNA sample present, i.e. can't possibly have been caused by transfer, then that is completely different.