As mentioned previously, dispersant acts like detergent to split up the oil into very tiny droplets, which should help speed up the natural detoxification of the oil and consumption by bacteria. Also, it hopefully minimizes big, thick surface slicks which then wash ashore and coat whole beaches with oil.

However, there are lot's of unknowns with the dispersant being used, Corexit. There aren't good studies on its environmental effects. Corexit's safety information from the manufacturer admits as much. The dispersants may cause genetic mutations and cancer in wildlife, so there is an argument that short-term damage may be minimized at the expense of long-term wildlife health.

I read one account which said BP has used a third of the world's total supply of dispersant, and this was fairly early in the disaster. Eventually, if BP continues at this pace, it's may have to switch to using even more toxic dispersants because the supply of less toxic ones are exhausted. Actually, who knows if that point hasn't been reached already?

BP is also using the dispersant in new, untested ways, like injecting it directly into the oil plume. No one really knows if that might have unintended consequences.

Another angle to the use of dispersants is public relations--since it breaks up the slick, it's harder to see the oil on TV. And besides that, there is continuing controversy over the control BP has over negative images--prohibition, arrest and fines for people taking pictures in public areas, threatening clean up workers who wear protective equipment like respirators, etc.