chaosmagnet: That's true, I thought about that too .

Jesselp: I wear a PFD where I am supposed to. I practice this mainly for situations where wearing a PFD is ruled out. For example open water distance swimming. If your muscles get fatigued and you get cramps, then this position is quite useful. Actually I have already had an opportunity to try this. I got pretty strong cramp in my calf while swimming in a pool. I could get to the shore without problem, but I decided to deal with it in the water as a training for such situations in open water. So I used this technique. I rested for a while, stretched the leg and resumed swimming. It worked very well. (There is usually someone on a boat near me when I swim longer distances though, but you never know.)

This could also come in handy for a lot of people that are carried away by rip currents. They try to fight their way back until they have no energy left and drown instead of staying in some sort of survival position like this. (You can escape the current by swimming parallel to the shore but not everyone knows it though).

I agree that in cold water it is not a good idea to do this unless you have to.