When I can't short CS to ground using only one SPI device?
Rising/falling CS line is not only used to enable the output of the slave, but also to reset its internal state machine. For example some flash memories have a following sequence to read: command_opcode address_byte1 address_byte2 and dummy bytes. You send as many dummy bytes as you want, the address within the slave is automatically incremented. How would you be able to send another command (eg. write) if there was no way to end the transaction? The CS line does exactly that.