The title actually says the main point of this thread.
Devs told us that they want to keep legendary items "legendary" as a reasoning for why they won't make legendaries transmoggable.
Right now every spec has the most legendary weapon they can get because the point of artifacts is actually that: get the best weapon you can get for your spec against the legion.
My question is: Why can't I still transmog my weapon to shadowmourne or tarecgosa's wrath when I'm carrying the Ashbringer, Doomhammer etc.?
Thinking that uniqueness of Ashbringer is not more important than uniqueness of shadowmourne is pretty dumb all in all.
Edit: Note that I most definitely prefer using Ashbringer's appearance over shadowmourne but for players who don't, this is kind of an unnecessary restriction.