Strictly speaking, they are not entirely useless. A multi-function device can expose multiple different buses, so it can add another child when gpiobus_attach_bus() is called. In an ideal world, we should have an explicit bus_attach_children() function at the end of each respective driver (especially if it exposes multiple functions).
However, this is only a theoretical notice , nothing more.

We could create a version gpiobus_attach_bus that doesn't call bus_attach_children for use by multi-function devices. That would mean all buses are added together for more control.

This looks good. No need to hold things up for Andy's suggestion, but I think I like it for future consideration.