The Vertical Video rationale, as explained by James Metcalfe, Executive Product Manager, BBC News Apps & Connected TV...
"We know, through audience feedback, there were several reasons people were not watching our video on mobiles: Most of our video has traditionally required sound, which can be a problem when viewing in public and much of the video was embedded in small screens making it hard to watch without rotating the phone. In addition, much of our video offer has so far consisted of longer TV pieces that aren’t ideal if you’re after a quick update and short of time.
"Our new vertical video addresses all those issues. It will be shorter than films used elsewhere on the BBC and tell news stories in a succinct and snappy way. In addition, all of our vertical videos are subtitled and can be used without the sound on.
"We have been testing this update and the videos with different groups of people for a number of months and the response has been largely positive."
Here's a an old-fashioned non-yoof orientated landscape short promoting vertical video (no chuckling, please..)