Uh, I use Dsmash a lot.
A) I love spamming the crap outta Dancing Blade. This punishes side-step dodging, so my opponents then begin to attune themselves to Roll Dodging instead. I've now conditioned them to eat a Dsmash.
B) If you are very close to your opponent, on mid sized characters and above, Dsmash will hit with BOTH slices. This FREQUENTLY owns people shield the first slice, go in for a grab, and get their toes chopped off.
C) Though this is a situational mindgame, I find that Fair -> a close Uair to -> Dsmash should not result in a shield grab. I need to test this more though. Many times, when I pop my opponent into the air with a falling Uair, they air dodge and land right on my Dsmash.
D) I also find Tipper Dsmashes to have a better "natural spacing" meaning that I'm frequently in that position, and I don't have to set up the spacing myself, like the USmash
E) Even on a competitive level, ALMOST EVERYONE ALWAYS ROLL DODGES when they are cornered at the edge of a stage, ESPECIALLY FD. Dsmash ftw?
F) After getting that shield worn down by Dancing Blade or Shield Breaker, guess what hits under!?!!?<4Dsmash!?!?!?!?
It all has to do if your good with the mindgames son. I love this move. It comes out quick enough to catch people by surprise too. PLUS, Marth's Fsmash DISCOURAGES short-hop approaches, and people KNOW not challenge him in the air. It's just another awesome move to use after you condition your opponent.
All that being said, I still prefer Uptilt in many situations though. It hits behind Marth as well . . . possibly has the best horizontal trajectory in Marth's arsenal if you do it right, and vertically finishes as well otherwise. Just as quick, less lag. Doesn't shield poke though, and doesn't punish Roll Dodge, but hits airborne opponent's as well.
Dsmash is situational, but HIGHLY underused.