I'm not sure I understand the argument. I can store moldy food in my fridge, in a container where it won't hurt anything else, but that doesn't make it good! At the very least, I can think of more appealing uses for 500 square miles than "landfill." Landfills also keep even things that don't need to be trash -- like food -- from degrading the way they could in other circumstances. Finally, making a lot of trash means that a lot of things whose manufacture has environmental costs are nonetheless being treated as disposable, and thus manufactured more often.

Styrofoam's specific trash badness has two aspects: first, it stays trash a long time (estimates seem to be on the order of several hundred years). Second, it can cause some water weirdness in landfills. Obviously other plastics are its co-conspirators in this last one, but when landfills get waterlogged, trash that breaks down nastily can get toxins into that water, which inevitably eventually overflows, getting crap into groundwater. Incinerating styrofoam is also not such a good idea except with sophisticated systems to capture the toxic gasses it releases; there are, of course, regulations about this which we can all hope that incinerators scrupulously follow.