It's a bit disappointing that this has been a prominent issue for over a year now, and no signs of a fix seem forthcoming. I wouldn't even mind if I had to set AirDroid to the default SMS app, because I like being able to reply from my computer and get notifications on my computer when I'm at home. Apparently I can't set AirDroid as the default SMS app, however.
This is really frustrating, and renders a very useful, likable app nearly useless because of the inconvenience it causes by sending the recipient and unnecessary duplicate message. It's annoying, and frankly not really worth it.
Maybe this will get fixed sometime in another year...maybe not.