A couple of things. (OK - more than a couple)
This is from a close reach / close hauled position.
If you're sailing in heavy conditions the reality is you're probably running deep reefs and shortened sails. Surely if you heave to immediately someone falls over the side, it is better just to let the boat get pushed down onto the MOB? Rather than risk crash gybes etc running back to him? Yes I get the cold water scenario and the need for speed but then the MOB should be kitted to withstand the cold as well.
BTW - downwind is a whole different story as you have to come about. But we still heave-to at the end.
The picture presented makes more sense to me if you're sailing off the wind - then there is a need to get back upwind and around the windward side of the MOB so that you can drift back onto him (as above)
Anyway this is all semantics, I agree that the best medicine is staying on the boat and I have many times in earlier discussions said that there is no place on my boat for ballet dancers who move around the boat like it was the USS Ronald Regan. On my boat in heavy weather, you don't leave the cockpit if there is no-one else around and when you do, you move around on your hands and knees. Walk, dance, pirouette from point to point and you'll earn yourself a solid rebuke, maybe even a tight slap when you return to the cockpit.
Sorry, a little off the point, yes I suppose crotch straps on harnesses will do some good but staying on the boat is better.