Such vest are not called lifevests but bouyancy aids. They do NOT rescue you, The idea behind these vests is that they allow you to float your face (mouth and nose) above the watersurface without excerting yourself with swimming or watertreading, even when packed with gear and clothing. As such you can survive longer as you cool down your body temperature less rapidly and don't waste energy. Basically your body will shut down all blood circulation to your limbs and only concentrate it to your torso. It is numbing and painful but you'll survive longer. It also allows you to survive when your muscles cramp up as, again, you don't need to watertread or swim to prevent yourself from dissappearing below the watersurface.

It is actually a swimming aid and not a rescue devise.

There is also a reasoning that too much positive floation or a real lifevest can be dangerous. These can pin you underneath your craft as it can be very hard to swim downwards against the provided floatation to clear any obstacles. I also think that the lack of croin belts is a design choice to be able to quickly removed the vest when needed.

Wouter