Why buy MELE if you own the originals with all DLCs?

I'm interested in what you're referring to as being "compromised" with the old games on the Xbox One.  All the Xbox 360 discs are completely backwards compatible now and have been for some time.  Comparatively, I would also now say that the old games perform better on my Xbox One than MELE does (i.e. fewer bugs and absolutely no crashes).  MELE has crashed totally several times on my Xbox One and some bugs are occurring with annoying regularity.  For example, ragdolled enemies failing to get up again such that I have to continually go looking for them in order to get missions to complete.. with some of them even sinking into the terrain, dialogue cutting out prematurely, and even squad commands for one or the other squad mate disconnecting mid-mission such that I can no longer tell that squad mate to fire a power or move to a specific location.  Sure, an exit and reload corrects this... but that means restarting the mission or restarting from a previous save.

Enemy spawns are now erratic at best... some battles feel like only 1/2 the enemies that should spawn do and sometimes one gets overwhelming numbers of spawns in rapid succession (e.g. waves that spawned in the old games can now either not spawn at all or spawn overlapping each other).

OP - If you're on the Xbox, I see no reason to encourage you to buy MELE unless something like a Photo Mode and graphical window dressing really appeals to you.  You will also sacrifice some "perks" to which you might have become accustomed:  1) DLC weapons and armors are no longer given to you at the start of ME2 and ME3.  You must purchase them (or in the case of ME2 research some) when you can both get to the shop that sells them (e.g. some are at Illium in ME2) and, of course, when you can afford them (e.g. the Valiant in ME3 now costs 250,000 credits).  2) Stasis is no longer available as a bonus power for Shepard.  3) There is also no access to ME3 multiplayer from MELE  4) There is no way to boost your endgame war assets if you desire either by using Multiplayer or by accessing the Galaxy at War website.  ME3 endings are now based on Total Military Strength only (e.g. you now require around 7,500 TMS to get the "breath" ending).