These chemicals spread the oil below the surface, making it much less visible and reducing the immediate economic impact. They could potentially also reduce BP's liability, not only by limiting immediate economic impact but also by inhibiting accurate estimates of the volume of oil spilled. At the same time, dispersing the oil deep below the surface could have major negative impacts on micro-organisms and other less well-known organisms essential for the functioning of the ecosystem. Are we avoiding short-term costs by imposing much greater and longer-term impacts on the biodiversity of the Gulf?
A good piece on tonight's PBS Newshour raises some of these questions, while detailed information on Gulf biodiversity and impacts from the spill is available here, among other places, and current fish & wildlife reports are available here.