Except that the big problem that caused this discussion was that when they were using the touch attack, it was only draining strength without damage, therefore making clerics with the negative energy protection spell invinvicible, since the shadows weren't even attacking.
Giving them an on-hit drain without a save throw, on the other hand, would be more fearful, that I can agree. It would at least force players to get their hands on negative energy protection potions / clerics before engaging any shadow. And, at the same time, it would still be a damaging attack, making negative energy protection nullify only the drain, not the damage they do.