A great roofing system should shed water quickly away from key areas that are prone to leaks. Roofers are skilled & trained to understand this. Valleys and transition areas where the roof changes angle or pitch are prone to leaks. "Nailers" do just that they install products usually quickly with air guns. These roofs fail prematurely... If your roofing contractor talks more about shingles, ridge vent, and nails then he is probably a nailer not a roofer.