To fight the enemies, heat and moisture, you must ventilate year round.
Heat in unventilated attics may cause temperatures to exceed 150 degrees causing damage to shingles, roof sheathing and possibly radiate to the living area. Moisture being the #1 enemy, causes rot, mildew, mold, paint blisters, and renders insulation to be ineffective.
Proper ventilation reduces energy bills, winter ice build up and eliminates mold/mildew which can lead to major health problems. By ventilating, you are extending the life of other building components, shingles, insulation, etc.