it's possible they have some issues with that such as parallax that has to be software corrected, and bokeh may look kind of weirdHave you watched Canon’s video on dual-gain sensor tech and what it can accomplish? It sounds to me like the answer to the current limitations. Video is down on the page a ways:
tricks they can pull on video are different than tricks they can pull on stills. if you played around with Canon's dual pixel RAW format, you'll see where the two raws don't exactly always align. With video and things in motion, this is less of a problem IMO.