Disc brakes up front with rear drums in the rear are adequate for the reason you mentioned, locked up brakes are locked up brakes, and I've locked up on at least one emergency braking situation (VW disc brakes front and 315-type rear shoes/drums.
I would qualify that rear drums are adequate if: (1) you have the wider 315-type 40mm shoes (rather than the earlier 270-type 30mm wide shoes); (2) the rear shoes properly adjusted (probably one reason for advocating disc brakes at the rear would be eliminating this aspect and eliminating a maintenance issue); and (3) you have braided SS steel brake lines (or great like-new condition rubber lines). If you have old rubber lines you'll be amazed at how replacing with a set of braided SS lines will stiffen up an otherwise soft pedal feel.
But for the low cost of adding rear discs, and if you are looking for a project that will reduce at least the need to adjust rear shoes, then why not. Just not a great performance gain.