Just found out that Little Mac D-Tilt has even better frame advantage than I thought--After a shield drop, it's -7 on block, and when well-spaced, some (I guess I don't know if it's most) Standing Grabs won't reach him, and all Dash Grabs are Frame 8 or later in Smash 4, meaning that Jab beats the most threatening/common response to a shielded D-Tilt, and with characters who have Frame 9 or later Dash Grabs (of which there are many: Bowser, Bowser Jr., Captain Falcon, Charizard, Donkey Kong, Duck Hunt, Falco, Fox, Ganondorf, Greninja, Ike, King Dedede, Little Mac, Meta Knight, Pac-Man, Palutena, R.O.B., Samus, Shulk, Sonic, Villager, Wii Fit Trainer, Yoshi, Zelda), a spotdodge also beats it and many other immediate attacks while giving you strong punishes afterward, creating a pretty safe mix-up that discourages opponents from grabbing or using immediate pokes to punish, helping Mac to deal with one of his great banes (grabs).
D-Tilt also has the advantage of putting Mac close to the ground, so it keeps him protected from many out-of-shield aerials.
D-Tilt also has an animation that lasts only 21 frames, so a well-timed D-Tilt seems to hit Ledge Jumps while giving Mac enough time to Jab or tilt an opponent who decided to Ledge Climb instead. D-Tilt's range and low hitbox height also makes it work well against some Ledge Drop -> double-jump options, so it may very well be one of the best ledgeguard tools for the number of options it covers reliably, rather than just because of its combo potential...and the knockback buff to his Forward-B in the most recent patch has great synergy with this; YOLO-looking Forward-B follow-ups will KO before triple digits if it's fresh and off the ledge like that.