Why doesn't the McBSP on OMAPL138 does not support the SPI protocol.

Because  C6748's SPI cannot write 24bit  word ,while the  McBSP supports 24 and 32 bit word sizes.On the 24.2.10 of TRM,  SPI Operation Using the Clock Stop Mode it says: The McBSP on this device does not support the SPI protocol.

I want to know why it doesn't support,and if I can adjust some parameters of McBSP to communicate with other TI devices supporting SPI protocol .

Thanks!