What's the advantage of DMA in byte I/O?
It's what DMA is meant to do.

From Blackfin+ Core Embedded Processor


  The baseline SPI peripheral is a synchronous, four-wire interface
  consisting of two data pins, one device select pin, and a
  gated clock pin. The two data pins allow full-duplex operation
  to other SPI-compatible devices. An additional two (optional)
  data pins are provided to support quad SPI operation. Enhanced
  modes of operation such as flow control, fast mode, and dual
  I/O mode (DIOM) are also supported. In addition, a direct
  memory access (DMA) mode allows for transferring several
  words with minimal CPU interaction.


Apparently yes.


  The SPI port’s baud rate and clock phase/polarities are programmable,
  and it has integrated DMA channels for both
  transmit and receive data streams.