A fair question, and I will attempt to provide an equally fair, and honest answer.
Unless I'm very mistaken, MAME's focus is on accuracy, sometimes at the expense of playability, where Daphne (and other emulators) focus more on playability, sometimes at the expense of accuracy.
Let's use Us vs Them as an example -
Scenario 1 - MAME and Daphne play the game equally well, with complete feature parity.
Advantage=MAME
Scenario 2 - Assume that Daphne uses some sort of trick or hack to make the game run at full speed, and the current version of MAME, in the interest of accuracy, causes the CG overlay to frameskip, or the preliminary driver causes audio anomolies.
Advantage=Daphne
Scenario 3 - Daphne's focus on LD games allows for specific focus on things like specific cheats for specific games, and fun things like a GUI based Virtual LaserDisc Player (VLDP), game trivia, and/or tips & tricks within the Daphne engine.
Advantage=Daphne
Scenario 4 - An unemulated LD is dumped into a LD-CHD format (let's say Star Rider). Someone on the Daphne team figures out a way to make the game work using hacks/tricks that would not be appropriate to MAME, but the MAME team is working on it. The game will eventually be playable in MAME, but is playable now in Daphne.
Advantage=Daphne, and eventually MAME
So what it really comes down to is can Daphne offer something to the player that MAME cannot or will not?
I believe that Daphne can differentiate itself from MAME in terms of features and focus. It's a matter of software evolution.
As to the question of format, I honestly find the quality of the LD-CHD format to be superior to that of the MPEG video Daphne currently supports, and a common file format is preferable to duplication of content.
Sorry if I appear to be rambling... I'm perfectly willing to discuss any aspect of this topic with an open mind...