Speaking from an engineering point of view - there are many seawater intakes in the average ship's hull. These are bringing seawater in to cool heat exchangers, water for pools, fresh water evaporators, etc. A lot of systems rely on seawater constantly being pumped in and out of the ship and that much oil could mess up a lot of equipment.
Of course, in most places of the slick it is only a few millimeters thick and on the surface. But wave action or simply the action of the hull going through the slick will provide enough mixing to send some oil close enough to a suction point. Ships often encounter and go through spots of oil, so I'd think the size and time spent in the slick is a factor.