> The worms converted about half of the Styrofoam into carbon dioxide, as they would with any food source.

I'm certainly not a chemist, but given that polystyrene is equal parts carbon and hydrogen (C8H8) and that carbon is roughly 12 times heavier than hydrogen I'd say that if the worms managed to convert only half of the polystyrene (by weight) into CO2 that looks much better than burning it, which -if done properly- would have converted all of the carbon into CO2.

However, I don't know how much energy can be harvested from burning it, and how to take that into account in order to decide whether it's better to burn it or to degrade it with the worms.

Plastic is inert in a landfill, but occupies a lot of space. If it's burned, CO2 is released into the atmosphere and toxic ashes are left behind, although way less space is occupied. If all plastic is degraded by these bacteria, apparently no disadvantages seem to be present.