Proper attic insulation requires an even layer on the floor of the attic to prevent the heat from the house from escaping through the roof. Secondly, the underside of the roof needs to stay cold and dry. Properly vented attics prevent moisture build up by allowing air flow to carry out moisture. Signs of inadequate ventilation include rusty nails, frost build up during winter or a musty smell.
Your consideration of proper attic ventilation may be the difference between a successful, long lived roof or a total failure in a short span of time. Considering the cost of re-roofing the average home these days, a few hundred dollars for additional ventilation should be considered a very wise investment.