Holding the smartphone with a half open book case cover whilst recording is less stable.
With a back cover the user can firmly hold the smartphone.
AFAIK there are no book case covers with the front cover being identical to the back cover, so, when front cover is flipped entirely, users can hold the smartphone in the same way as a back cover, but the front cover does not cover the camera.
I.e. users can can record videos and/or take photos the same way as with a back cover.
Is this a strange idea then?