keeping things
like the DAA instruction intact and supporting a bunch
 of do-nothing instructions like MOV DL,DL. 
 FYI, that most certainly doesn't
just "do nothing."  It updates the flags
 based on the contents of the DL register, and you could do a conditional
 jump based on the results. 
 
NO, it doesn't.
But OR DL,DL  or AND DL,DL would.