This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
The only reason why you would want to enable this option is that DMA modes are less likely to corrupt data than PIO modes. There will be no difference in CPU usage. (when DMA/33 arrives, the improved bandwidth will be another reason).
Unfortunately, at least one user has reported a drastic decline in drive throughput with DMA enabled. The reasons are unclear, so YMMV.