While it isnt strictly nessesary to add these i think it would still be a nice thing to have as doing something like

looks weird and is also tedious to do for every Input Pin.
(also considering most components take in a Binary signal anyway.)

As for balancing The Digital I/O Pins could could equal the same amount (or a bit more) of resources it would take to make one as shown in the image above.
ie adding the cost of a normal Analog Input and Signal Digitizer, plus a bit of extra wiring, either making the Digital I/O Pins as big as the Signal Digitizer in size or somewhere between Analog I/O pins and the Signal Digitizer.

PS: After having written all of that, i think only Digital and Analog Inputs make sense. Since there is actualy not realy a point in having a Digital Output if the next logic board probably has Digital Inputs anyway.