WAAS was created solely to increase vertical accuracy. For routes in the horizontal plane, it only increases accuracy by a few (less than 10) meters, and does not allow any increase in routing precision. Even without WAAS, GPS is more accurate than ILS in the horizontal dimension. Use of WAAS does permit the GPS to warn of obstacles or not, based on altitude. It was created to allow precision GPS approaches, which are not possible without it, but does little for horizontal accuracy. If the routes are TERPed beforehand, WAAS isn't necessary, but if the routes are going to be flown without being previously checked, then it can help avoid obstacles, as long as the database is completely correct, and nobody has erected anything since it was installed in the GPS. I don't trust obstacle databases completely, at least not enough to bet my life on their accuracy.