The idea with the visage form was probably it was supposed to be the unique ones like what Alextrasza has, as compared to just the "You look like another race" that most dragons pull off
Price we pay for all the customisation options. When DKs got added to the allied races and Pandaren, they didn't get any fancy skins to go along with it unlike the standard or Cata races. Demon Hunters seem locked to BElves and NElves for a long while simply due to all the customisation they got. We get all sorts of crazy stuff for Drac'thyr because they want them to stand out as their own thing and be visually There.I dont get why this can be defended. It was such a clearly bad decision that was immediately called out as nonsense yet here we are, such a terrible choice being defended.
If they could change into any other race instead, you lose a lot of that draw of 'hey look at the new race, don't you want to appear visually distinct and roll a new one', plus we wouldn't get the sheer variety of customisation we got. You'd at most get the dragon eyes and horns, and that'd be that. "You get one distinct form that can't wear armor and the other form is a race that you've had unlocked already, with basically nothing new except for horns (which any DH or warlock is already laughing at because 'you have horns now' is not a big draw)" is a lesser draw than "Here's two completely distinct and new forms"