I have a different perspective than Q.

Oil is dispersed to reduce concentrated product on the surface to protect marine life on the surface (I suspect NOAA, who was part of the operation with EPA), had concerns for some highly productive areas) , which is very important for adult and developmental stages (i.e larvae) of sealife (i.e.and seabirds...as well as primary production phyto and zooplankton) It also reduces movement to the coast, which is also an important place for marine life and economic interests. Breaking oil into smaller droplets dilutes and enables bacteria to break down the toxic elements. Just because it is in some plumes under the surface water column,   does not mean it has not been dispersed into smaller droplets. If it had not been dispersed, I suspect one would have seen significant and immediate effects on marine and coastal life. The longer waves and currents have to dillute and disperse it, the less liklihood there will be as serious an impact as if it were left concentrated on the surface. I suspect the direction of surface winds was another consideration, at the time.

Whether the correct dispersant was used, is an entirely different matter. Additionally, you do not release this much oil and dispersants (the former more serious) in one place over an extended period without significant impacts, short and long term.