No way in hell does DJC take "just one frame," especially if the goal is to get anything out of it.
-How long does it take an aerial to come out?
-How many frames from where ness is currently until he hits the ground?
-How many frames of jumpsquat do you have to wait?
If you jumpsquat for 4 frames, Double Jump on the fifth frame, then do an aerial immediately, Ness lands without doing anything. There's a wait period between starting the Double Jump and canceling it with a move that must be adhered to or else the move won't come out before Ness simply lands on the ground. Even more of a wait period if you want to DJCFFL the move. When you have to dedicate 10 frames to an action before you can even start a move, are essentially floating in place in the meantime, and Marth's magnet hands grab you from a very vague general area on frame 7-8, it isn't hard to see why it makes it more likely to be grabbed. Ness's DJ goes DOWNWARD for the first frame or so for god's sake. Depending on what point in the DJ you cancel it, then, there is a small wait period before Ness starts to fall, as he'll be rising for a frame or two before being given the option to fast fall. See also: Mewtwo's extended Double Jump
DJC makes your aerials land lower, it doesn't make them any faster to come out and it certainly does not take just one frame to occur.
But, like OP, I'm just not using the tactic in the situations I should, and Marth already happened to be doing the grab anyway. And for future reference, this is the kind of thing I'm talking about when I say I'd rather explain things in person, because the concept is absurdly difficult to communicate through text. In person I get to act out Ness's Double Jump with my hands :<