  [stuff deleted]

Actually, it does make a reasonable amount of sense. Fixed disk
sectors are buffered by the controller, and transferring them to
memory with a 'rep insw' (or whatever the instruction is called) is
quite efficient (single instruction, goes as fast as the
controller/cpu know how to use the bus). Since the 286 wasn't cached,
the bus is likely a critical resource relative to CPU performance, and
it's possible that DMA bus interference would cause as much or more
loss of CPU cycles (for 'computing') as does the 'rep insw' sequence.

The floppy, on the other hand, is not buffered, so that using the CPU
for floppy data transfer (as was done on the PC Jr, by the way) really
does stink.