OPEC has never really been a "cartel" in the conventional sense of an organization that agrees to restrict output to maximize revenues. So its decision on Friday not to cut production was entirely predictable and the only practical option open to its members.
The strategy of maintaining production even as prices fall, led by the Saudis but now more or less embraced by most of the organization's membership, is really the only sensible course.
Most traders sense this: The price of Brent for delivery in December 2015 has been virtually unchanged since February and barely moved on Friday.