Ice dams are problematic in many homes in cold climates. They are typically caused by either inadequate insulation or more frequently, poor air sealing. By recommending ventilation to a homeowner you’re saying that you as a contractor are unable to air seal and/or insulate their attic.
Don’t treat the symptom (ice dams) treat the problem (air leakage).
Oh, and the worst culprit are roofers who recommend Ice and Water Shield. They must dread winter when they get the phone call in February telling them that the nice new roof they installed last summer is leaking?????