Turns out that bacteria at the bottom of the ocean love the sludge that's so toxic on land.
So a big "yay" for PR and feelings over facts.
Also, assuming what you say is true, was this known at the time? Because that would seem to make a big difference -- opposing dumping "sludge that's so toxic on land" into an environment where you don't know the effects seems to be more than "PR and feelings", but rather caution and prudence.
"Writing in the journal Nature in June, Professor Euan Nisbet and Dr Mary Fowler of London University argued that the quantities of heavy metals in Brent Spar are minuscule compared with those found naturally in parts of our deep oceans. In so-called "black smokers", whole communities of highly specialised bacteria eke out their lives where the earth's crust vents huge amounts of superheated water and metals into the ocean depths. One estimate puts this discharge of metals worldwide at 700,000 to five million tonnes a year.
Dump the Brent Spar near such a vent, argues Professor Nisbet, and its impact could not even be measured. Its contents might even give the bacteria a free meal."
http://www.independent.co.uk/news/science/to-dump-or-not-to-...
Also, the amount of oil was much less than Greenpeace had stated, estimated at somewhere around 100 tons, rather than 5500 tons. And oil is something that bacteria do eat.
All the articles I could find essentially state that Shell's plan was scientifically valid and posed less environmental risk than disposal on land, but didn't "feel" right.
Another more detailed look: http://negotiationlab.co.uk/user_uploads/KSG%20Brent%20Spar....