Originally posted by Mike Cash Wouldn't that only be useful for TTL?
Not necessarily. TTL (and p-TTL) uses the light sensor(s) in the camera body. The flash's light sensor is not used thus where it is located or what direction it is facing doesn't matter.
Originally posted by Mike Cash The key distance is the flash-subject distance and I can't understand why an auto flash sensor would benefit from being mounted on the camera. Camera-subject distance is irrelevant and there's no real need for the camera and the flash unit to communicate.
That is very true. But in general, especially when you hold the flash with one hand and the camera with another, the difference between the flash-subject distance and the camera-subject distance is negligible. With the light sensor on top of the camera, facing the subject, you can aim the flash head anywhere, e.g. backward to bounce off a wall behind, or upward to bounce off the ceiling, and still get decent exposure. Note that the most popular Vivitar flashes (283 and 285) can not swivel.
I think Sunpak had the best design with the dedicated modules. The modules can be shared by different flash models, from hotshoe mount (433D, 444D, ...) to ring flashes (DX-8R, DX-12R) to hammerheads (555).