Am I missing some obvious downside or exploitability? This would be awesome. Imagine controlling the camera as you watch games in spectator mode following your favorite player or getting the best overhead view of the whole battle. No need to worry about encoding quality balancing resolution and fps. No more braodcasters switching views on you even though you know they should have stayed on that dps who was about to make a play. Want to really analyze an engage so you know how every angle played out? Viewers having control of their own spectator mode would be amazing.
Still I think it wont happen. You still want a video broadcast for people watching on tv and phones or who want the camera controlled for them while they relax. Splitting up the broadcast format could be seen as diminishing the value of each half and I think that the OWL owners will have a soft spot for traditional broadcast techniques along with how lame it would be to have ads injected into the in game spectator mode.
But if they did it, for many it would bring watching overwatch competitions to a whole new level.