I discard the veggies before reducing the sauce for a couple of reasons. First, the veggies are cooked for so long that they basically turn to mush, and all of the flavor is extracted. Second, I love a silky, smooth sauce, and the best way to achieve that is to discard the solids. Other readers have used the veggies in the sauce - instead of straining it, you can use a stick blender to puree the sauce with the vegetables in it. It just depends on the texture you prefer.