Well, the move is also just hard for me to consistently get out. I'm someone who can't stand tilt jumping, and running, and then holding down, half jumping, and immediately hitting down+a, does not happen all the time. The way I can most often do it, is actually hitting X (jump) with my index/middle finger, and then using my thumb on the c-stick to hit down as soon as I jump. I'm getting a little better at it, but it's just real tough to hit opponents with it, since it certainly is no approach. Again, I think it would be most useful for punishing, especially if you can get utilt out, which can be a kill.
Although now that I think about it, if they were at a high enough percentage for a kill, they would bounce pretty high from the dair. I suppose it's a decent amount of damage anyways...
Oh and, thanks for the sig comments
I drew it up in flash (I'm actually working on a full cartoon, won't be out for a very long time though).