Reason to Add A Soffit to a Shower
I think you would want the shower ceiling to be the same height as the rest of the bathroom especially with 8 ft ceilings.  An average height man with raised arms would touch a 7 ft ceiling. Who wants that? Everybody wants higher ceilings  rather than lower.

The only possible benefits of a 7 ft ceiling might be to better confine water vapor and mist to the inside the shower enclosure during showering and to reduce the amount of tiling if the tiling goes to the ceiling, but I don't think this is enough benefit to balance the cramped feeling of a lowered ceiling in the shower.