Only problem is if you actually wanna use footage with sound... I guess y ...
I'll bet it would be dead easy for them to get the app to record sound in sync with the video. Even if it only saved on the phone/tablet and not back to the SD card it would be great.
Think about it, your phone/tablet will never be more than a few inches from the camera so its mic should be fine.
Waiting.... every other company has offered their hand held units now and have them on the market. DJI was first to announce this great idea and has dragged there feet.