Is there any real reason to play the DS remakes over Mystery of the Emblem?

[Glaceon10]

The Nintendo DS remakes have multiplayer, if that interests you, and the multiplayer was actually quite good, to my knowledge. You cannot play the online multiplayer any longer, though you can still play Local.

Also, the chapters which you mentioned in which you must kill off units to obtain are completely optional, and the characters which you can obtain are intended to be replacement characters for particularly bad players. All of those characters are new characters and no previously seen characters in Dark Dragon and the Blade of Light are locked to those types of chapters. The exception is Chapter 24x, and you must obtain it by having Tiki dead and the Falchion not in your possession, though due to an exploit which you can find (which I do not want to mention due to spoilers), you can still complete the game with the good Falchion and Tiki.

Also, Book 1 of Fire Emblem: Mystery of the Emblem has some cut characters and cut chapters, presumably due to cartridge limitations of the SNES. Not only does Fire Emblem: Shadow Dragon have all of the chapters originally included in the game, yet it also has the aforementioned Gaiden Chapters alongside Prologue chapters, all of which are completely new and add extra story.

Also, the game, in general, has gameplay improvements. You can now skip enemy turns, mark enemies and enable the danger zone similar to Fire Emblem Awakening, and there is a pre-chapter narration before every chapter, similar to games such as Fire Emblem: Blazing Sword, Fire Emblem: The Binding Blade, etc.

However, for Fire Emblem: New Mystery of the Emblem ~Heroes of Light and Shadow~, there were many more changes. There are now Gaiden Chapters which you can actually access by not killing off units, which I appreciate more than what was seen in Fire Emblem: Shadow Dragon. It also has most of the gameplay improvements which Fire Emblem: Shadow Dragon implemented, though whether you want to play this game or Fire Emblem: Mystery of the Emblem (Book 2) depends upon what your opinion of Fire Emblem Awakening's Avatar is. If you hated My Unit in that game, you will most likely hate My Unit in this game. They are remarkably similar, and the game shifts its focus from Marth to Avatar often (from what I have played, at least), and many detractors of Fire Emblem Awakening's Avatar (such as myself) will most likely find Book 2 of Fire Emblem: Mystery of the Emblem to be more fun.