No that's the difference between an art issue and a sound issue. If you've ever had the misfortune of running with a hunter with a gun you know that sound.

*BANG, BANG, BANG, BANG, BANG, CLUCK, BANG, BANG, BANG*

It's one of the more annoying problems in the game. This simplifies it for the players.
However changing MH/OH causes and issue where there would have to be reversed art for the exact same item (fist weapons would be the prime example of this, of course,) since not all the off hand items are the exact same from both sides.

And while it could be as simple as going in and flipping the OH weapon in the system, considering how many OH there are, that would take time, and require a larger patch overall, since the client side machines would have to be updated (more than likely.)

Considering how fast they're putting this out, I'm sure they wanted to simplify the process as much as possible to begin with, and then expand from there. So we may see MH/OH transmog, but they've given their reasoning as to why they won't do that, but will allow Bows, Xbox, and Gun Transmog.