Why does the life jacket only have one D-ring?

Sometimes a lesser quantity is better. One D-Ring gives additional safety because you are always secured when you hook on to this one ring. With two-ring systems there is a danger that in the heat of the moment one only hooks on to one ring and the system is then dysfunctional because it is not using its total strength. With SECUMAR, the single D-Ring and the central anchor buckle are to be viewed as a unit which provide the necessary strength.