The major argument for genetically modified foods is that through modification it becomes possible to grow suoer-sized varieties of certain foods as well as to grow those foods in areas that otherwise would not be able to produce them because of challenges from environmental factors. Genetically modified foods can also be created to offer more nutrients in smaller portions. All of these factors are critically important when we consider the degree to which starvation affects a large portion of our planet. Creating large quantities of nutrient rich food easily is something that scientists see as a potential answer to world hunger.
On the negative side, however, we don;t really know what genetically modifying food products is doing to our bodies when we consume them or to the environment itself. It;s like the butterfly effect - what we do to one thing may have ramifications that are felt thousands of miles away or a century later. Genetically modified foods seem safe NOW but we don;t know what effects they might produce later on down the road - babies with birth defects, land that is no longer able to support intended growth, unintentional cross pollination that destroys existing plant life. The list of negatives is as long, of not longer, than the positives. However, as with any growing science, it comes down to whether or not it seems safe enough to be worth the risk of some potentially negative outcome that we have no way of predicting.