Pivotal | Cruising the California Coast 

It may be optional for normal operating conditions and may be critical for redundancy. For example, if 3 of the motors, rotating clockwise are lost, then there will be 1 CW, 3CCW motors will be able to lift vehicle but unbalanced torque would make it spin like a helicopter which lost its tail motor. That elevons would combat that and create torque for redundancy. It may also help stand still while vehicle is not parallel to surface or smooth transition from hovering to Forward flight. Another thing is, it is not an unmanned which means there is a human inside the vehicle. Changing angle every other second to hover in place would not comfortable for pilot, therefore, instead of leaning forward a split second for backward trust for position correction, elevons may be used. I am a employee candidate who has a technical interview tomorrow for HW group :)