I'm pretty sure that on average DED gets more kills than Marth's DB by a solid margin actually.
Not
really. All of Marth's tipper hits have much more launching power than Roy's DED.
Roy's DED is designed to link consistently; this is why his sourspots drag opponents closer to the sweetspot. To compensate for this, it has lower damage, and thus, launching power.
Roy's DED gets KO potential approximately 15% later than when Marth's can KO. What's more, Marth does easy setups into his tipper (it's amazing how much people overlook down 3 into neutral 4).
On a somewhat related note, something else I find is overlooked is how using it at the ledge extremely benefits Marth's tipper mechanic. Essentially, when doing Dancing Blade or DED, each swing makes the character take a step forward. This essentially means Marth will likely not be landing his tipper finishing variants until around mid percents onwards due to knockback growth etc (this is why the 3rd down hit links perfectly into the 4th neutral finisher, as the knockback is enough to launch into that area). Using it near or at the ledge helps this in more situations than you'd think, as the ledge is essentially a stop point for Marth; he can continue using the move, but he won't go any further forward. This allows him to tipper his finishing hits easier than before, and this is only made better by the fact that they can KO at this range.
This is a good example -
http://www.youtube.com/watch?v=SFS59DgNvSs&t=9m34s
Had Marth had more space to step forward, he would have landed his sourspot, and it wouldn't have KO'd. Marth had nowhere to step, and this benefitted him a lot.
In the situation he had more room to step forward, the upwards finisher would have likely tippered at that percent. Not sure it would have quite KO'd, mind - perhaps 10-15% more and it should.
I also want to continue on and say that neither Dancing Blade OR Double Edge Dance are "busted" in this game. Smash 4 has struck a fine balance with these moves. The moves link consistently, and have use in each character's moveset, but also aren't incredibly stupid by design. Brawl's Dancing Blade was what I would call closer to "busted"; a frame 4 move that linked incredibly well even up to 150%+, and had KO power on both sweet and sourspots at percent ranges that didn't make sense.
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Another misc thing, totally not related I want to mention, is that Mr E only got that last KO as he did because of how ZSS'd dash attack extends her leg behind her (check 13:21). Had Mr E been a frame or two later, her leg would have not been stretched out as far, and the Fsmash would have whiffed. I love analysing things like this.