I do not know about those numbers about Nadal I guess if you count them Ill believe you. I think you are also right about the progression thing, it may be an evolution due to ball coming faster and the players need to return a good ball with enough clearence and pace, but I think it is not made on purposse, they do not think: "oh here comes the ball, Im going to do a reverse forehand" the same way they think, "im doing a slice".
I know many pro players are doing it, even Fed is. But I just dont think it is a stroke that is trained, and it shouldnt be, because hitting the ball that late will certainly hurt young (and old) people arms.
Well, first the numbers were an estimate based on watching him play, and not just a concrete figure. In addition, I think you're misinterpreting what I mean by 'natural progression.' It doesn't have anything to do with the ball coming faster, or returning balls 'late.' In fact, if you watch Rafa and Fed hit these reverse forehands, if they're set up, they still take it quite far in front of the body.
What I meant by it being a natural progression is that we've all been taught to hit from low to high to generate topspin. Classically, players are taught to finish around their opposite shoulder, thereby ensuring a low-to-high swing. As we all know (or should know), the more emphatically your racquet travels vertically, the more topspin you will hit (same goes for the racquet travelling horizontally and pace). So, coming back to the whole natural progression angle, the reverse forehand is basically the topspin equivalent of a flat swing. You're swinging the racquet upwards so drastically that, beyond even finishing by your shoulder (which few players do now anyway), you're finishing by the side of your head - the same side of your head (hence, I believe, why it's called the 'reverse' forehand, and not because of the racquet reversing its direction as Gorilla would like to believe).
In actual practice, the reverse forehand is more effective than it would seem. The sort of whippy arm action that's required to achieve a lot of racquet speed with this shot (since it's hard to use torso rotation to power a shot which doesn't actually come around your torso) applies a very deceptive amount of pace to the ball, as well as the extreme topspin you would expect.
Here's a pretty good clip:
http://it.youtube.com/watch?v=oeXRYb-B3OM
Notice how far in front and 'unlate' he takes both balls.