Ben, I'll tell you why you haven't seen this shot used much before-it's not logical. This isn't a bad thing though, you'll get a few free points trying it because the other guy will be so surprised, or he may not be good at ripping slow high balls, it depends on what level you're playing at. I'd say some 4.0's and up will punish a moonball approach more times than not. Obviously, it gives your opponent a nice angle to rip the ball down at your feet and really get some pace on it. That said, if you try it and it works, keep doing it! It's kind of like the lob approach, it makes no sense at all, and based on that, it's going to be genius, or horrible depending on the situation.
Proudly donning the Nadal avatar until Fed either wins Roland Garros, or serves Nadal a bagel.