Well, I mean, I don't use it as much in a pure jump -> Fair combo, if that's what your original question meant, because you're right, that the risk is too high for the reward. But there are other scenarios where it can come up, not the least salient of which is "your opponent is falling from an earlier move you performed on them."
I do favor the Nair as a way to add damage to someone in the air, but if I want to try to KO, I'm gonna try it. I don't guarantee I'll hit with it.
There's also "[hitbox of Dair just isn't in right position and] you're falling on your opponent, who (usu. in virtue of you not doing this often) is expecting you to air dodge rather than use this (or any) attack." The prior sign of this case, tends to be your opponent coming right after you in an uppity manner. I suppose they're either just aggro, or they don't expect you to think of using this move (maybe they're intimidating you into air dodging by reflex).
The point here is I use the move, because it is distinct from other moves in relevant ways, and it has no strictly better alternative.
The point of my first post is: Your question was equivocal. How could it be answered; does it make sense to ask it? I could only think of trivial answers - and the best way I thought to make you see that would be to actually make the absurd reply.
Does that make sense? So what information were you looking for?