JMO...If the water and ice shield is not wrapped over the edge of the sheathing onto the fascia the ice dam will cause the water to go wherever it wants. That includes upward between the water and ice shield and the sheathing or under the sheathing into the soffit and even into the interior of the house.

After all the years since I built my house I have not had any problems . Since we have began to get rain in the winter I have had the glaciers form on my eves and since I built the house before water and ice shield was around I am getting ice/water in the interior of my house. New roof coming next spring!