It's because by description and possible use cases Dillon mentioned, it's the only possibility:

- You cannot use additional battery without physical connector to the device
- It's certainly not an easy task (alhtough it's possible) to create additional flash for camera without physical connectors

If I remember correct, Dillon actually confirmed this in some interview. The concept is very interesting, but it's likely that Jolla has to create those other halfs by themselves. At least in the beginning.