Both fares have NO HIP OR MILEAGE EXCEPTIONS APPLY rule in EF.
The presumed benefit of the "D" fare is the unlimited stopovers (in comparison to the I fare's 3 allowed of which only 1 is free), in conjunction with free changes and free cancellation. I suspect, however, that virtually any planned enroute stopover will trigger an HIP re-faring (to a higher fare).
I hope that collectively we can find that I am incorrect.
