The thing is, they don't have to be approaching from underneath the stage...how often do you use your Dsmash to edge guard? If you connect with a Dsmash while they're trying to recover (from any direction or angle), they're held in place for you.
As in...you can just dair them. The down B spike is better at lower percents I think, but this still seems useful.
Sorry, I didn't explain myself very well. I was trying to explain how to exploit the trajectory of the d-air so that you can spike someone below you and still land on the stage. They can be at any height really, so long as you line it all up. Kinda like this ugly diagram (the //'s are the trajectory - ignore the dots: they are placeholders):
.....................................(ZSS) (d-air facing the stage)
......................................
//
.....................................
//
.................................(Opponent)
....................................
//
...................................
//
..................................
//
..................................
V
==================
======LEDGE========
==================
I see your point about using it in conjunction with the dsmash. It's theoretically useful, but they have to be at a decent % for it to kill them (unless they have horrible recovery), in which case, they are ripe for a down-B spike, which is easy enough to time out of a d-smash, so long as they are in the right position. And if they are over like 80-90% (or 70% if they are a light character), a second dsmash -> b-air will kill them anyway if they are at the edge of the stage.
I'll give it some practice with the d-air though, as learning never hurts.