Akita, I hear you but for the sake of a few pounds the costs saved is fairly minimal!
The price of the Battery grip in the U.K. is £179 which In my eyes is very expensive for what it is for what is essence an inert lump of plastic! The possibility of a useful battery/vertical grip doesn’t stop the Z6 being used as a video camera,
Mike, the MB-D18 for D850 has a joystick, command dials and an AF-ON button that controls the camera in real time. So, the design should require the considerations not only as a power source but also as a remote control, as Ilkka points out. That should also raise the bar for the reliability of the connectors.
On the other hand, MB-N10 for Z6/7 is designed to work just as a large battery, and the connector is just borrowed from the EN-EL15.
Admitting that the actual price of MB-N10 would be a bit steep, the difference is much more than just about the cost of the parts.
I do believe the primary application of the Nikon mirrorless cameras (as well as the D780/D850) continues to be still photography, with video as a secondary use, but with increasing importance and consideration in the design. The camera's ergonomic design is still mainly stills-optimized, even though some aspects are now optimized for horizontal shooting (lack of vertical grip with controls & tilt screen only tilting in one axis).
However, as surprising as it might seem, vertical videos are now standard in some social media platforms such as instagram and facebook, even to the point where it is enforced (i.e. on facebook if you shoot a horizontal video on a mobile phone, the platform rotates it to vertical). I believe camera manufacturers should include consideration for this as well. The vertical videos should not be as elongated as horizontal videos are; the aspect ratio should be less rectangular. And the vertical grip becomes useful for video as well if it is considered as a valid option for usage.
Ilkka, the need for the vertical video is mostly for the lower quality data like that shot with a smartphone. It is true that, for example, the German Bundesliga started to spare some cameras for the vertical setting for those who watch the games on the smartphones. That's why I didn't say "never".
I would agree that the design of the mirrorless cameras are still-centric, but the videographers and cinematographers who use these cameras are professionals or high-end enthusiasts who require high quality data like DNG RAW, ProRes RAW, Blackmagic RAW, etc, and they typically mount the camera into a cage to attach various necessary accessoriies. So, the design of the camera itself is not essential and can be left as still-centric which is beneficial to a gorilla-style shooting for video as well.
