With the way it's used, his Dtilt is pretty much the whiff punish sweep you'd find in games like The King of Fighters or Street Fighter where it has good range, sneaks under hitboxes since it hits low not mid or high -- it's a Dtilt after all --, and hits hard. Other characters have Dtilt's like this, but they're usually not used like this for whatever reason. Anyway, compared to his Ftilt, it's just as fast -- only 1 frame slower --, has just as low of a recovery -- 19 to Ftilt's 20 --, has the same active frames, 3, and has similar range. Its edge over Ftilt is that it's safe on-hit wherever you hit whether it's the 9% or 11% sour-spots or the 12% sweet-spot and at 0%; Ftilt isn't safe on-hit until 40% on Mario. Ftilt's edge is pretty obvious; as Dtilt is a low-hitting move, it's locked to only hitting people on or near the ground while Ftilt can hit mid and kind of high normally and being angle-able, can hit high and low. Functionally, they both poke, however, Ftilt's strictly for poking while Dtilt's vertical angle allows it to set up. As people have said, his Dtilt is versatile functioning as a poke, punish, and set-up.
Falco's Dtilt is arguably his best tilt and some of them are for the wrong reasons. One of them I already pointed out: the disjoint. It's not a small disjoint like on Marth's tippers or something, but a noticeably invisible area where he hits you despite you believing you're not in range. That's just unfair and Falco's got another one with Fair's landing hit which itself shouldn't have existed because the landing animation having a hitbox makes no sense. That's another story for another day. Now, the move being a disjoint itself means its difficult to challenge with the disjoint making it even more difficult since there's extra, invisible range. With the disjoint, Falco's Dtilt has about as much range as a sword user's while other characters like Bayonetta, Diddy, Greninja, Mario, and ZSS have shorter, more "melee" range. Granted, I do think Dtilts should have more range, so people can use them like a sweep, but not the extent of Falco's rivaling a sword user's. Otherwise, if they're short-ranged, then give them something like Diddy or Wario's where it's a fast or low recovery poke like a crouching light kick e.g. Ryu's light Dtilt which is his cr.LK from Street Fighter. Anyway, I digress.
The other reason: it's functioning more like what Fox's Dtilt used to be. This part I blame on Brawl's change to Falco's Dtilt where they added a 9% sour-spot to its tip and significantly lowered its knockback not adjusting its knockback to work with Brawl making it so gravity affected vertical knockback so that vertical KO moves didn't have to have ridiculously high knockback to KO. Dtilt went from having 13% hitboxes covering his entire tail to 12% about 2 thirds of it and a 9% sour-spot tip and it went from having 125 growth to work with Melee not factoring gravity for vertical knockback to a drop to 90 growth in Brawl with the base not changing at all -- stayed 25 in Brawl. Keep in mind that Falco's Dtilt during Melee and Brawl had shorter range than Fox's whose tail is not only longer, but stretches more than Falco's, especially in Brawl. What once was a short-ranged, but powerful and versatile Dtilt that KO'd ~100% became a short-ranged and weak Dtilt that KO'd at ~170% sweet-spotted. It was still versatile as Falco could set up with it, but with the changes to hit stun and hit stun canceling, it wasn't as useful. Although Smash 4 made it stronger by adding more base knockback with the sweet-spot having 50 and the sour-spots having 35, it did decrease the growth on the sour-spots by 2 to 88. In adding another sour-spot, the 11% hitbox on his tail, it still had the problem where it's weak; 11% hitbox KO's at ~176% and the 9% KO's at ~206%. Dtilt only KO's if Falco's right next you since the sweet-spot is on the base of his tail, but it's not really that strong, KO's at 136%, considering that you have to be that close. The 3 different hitboxes makes it inconsistent if you're looking to do one thing. Yes, each of them can set up, but in the times where Falco punishes someone with it, people are popped up high, and nothing happens because he wasn't right next to them. In contrast, Melee's Dtilt which having lower base and being in a different environment, it could set up and KO consistently. It was never a you hit with Dtilt and question if you hit with the 11% or the 9% hitbox hoping for that high percent KO to close out a stock or a need to get so close to KO an opponent when a safer option like a spaced Bair would have worked. Right now, it's this weird hybrid of Melee Falco's Dtilt -- only the sweet-spot -- and Melee Fox's Dtilt -- the range and the weaker hit.
Regardless, Falco's Dtilt is a good move, but it just doesn't seem "Falco enough". With it being a disjoint, its range and disjoint, and versatility, Falco's Dtilt is a good move, however, what hurts it is that by having 3 different hitboxes with the sweet-spot being the smallest and located near Falco's body, it is inconsistent, especially if you're aiming for a Dtilt that sets up at low to even high percents and KOs well. In a way, you could argue because of the different hitboxes, it has very short-range, shorter than Roy's sweet-spots, if you're aiming to KO, but average to good range with its 11% hitbox and 9% disjoint. Because of that, I don't think it should be higher than B as despite its versatility and range, there are better whiff punishing and/or setting up like Captain Falcon, DK, Ganondorf, Ike, Lucina, Marth, Mewtwo, Ryu, Samus, or Shulk's. Of the characters I listed, most of them have more consistency like Captain Falcon whiff punishing you with his Dtilt will always do 10% and send you at a horrible, shallow angle. Only problem with Capt.'s is that it's slower and not a disjoint or disjointed.
Links: Brawl Falco frame data:
http://opensa.dantarion.com/autoframe/FitFalco.php.
Melee Falco frame data:
http://dx.smashbr0s.com/chara/cters/falco/.
Either low A or high B it's a very nice move. Fairly quick, has decent range, can be used as a combo starter a effective mix up if you can drag airborne opponents down to the stage with Nair. It kills and its a good move to shield poke high up shields.
