I dunno about you, but I find it really easy to believe that the SAMD20 sports a DMA controller by glancing at this page: http://www.atmel.com/microsite/samd/
Well, it doesn't... Check yer datasheets, always!
DMA is convenient for the logger, if the system should read from the ADC at an exact frequency while the CPU is free to write to the memory card or do anything else. Plus, both the DMA and the ADC can run in sleep mode, so that they can work together to fill up a buffer, and then wake up the microcontroller to write the buffer to the memory card, thus conserving battery energy. The sampling and writing could be solved by using a FIFO buffer and pausing the write to put a sample into the buffer, but DMA does this elegantly and saves power at the same time.
Save the DMA for the next revision. I'll try getting by without for now, or solder up another board with a DMA enabled D21 instead.