From the A4988 Data Sheet:
As to being useless, I'd have to defer to the electrical engineers at Allegro in their design decision on why MS1 and MS3 need a 100k pull down resistor and MS2 needs a 50k one. But based on their design decision, my guess is it IS useful and that those resistors are just pulling the line low to ensure they are low unless the jumpers are enabled to set them high.
Quote
Microstep Select (MSx). The microstep resolution is set by
the voltage on logic inputs MSx, as shown in table 1. The MS1 and
MS3 pins have a 100 kΩ pull-down resistance, and the MS2 pin
has a 50 kΩ pull-down resistance.
As to being useless, I'd have to defer to the electrical engineers at Allegro in their design decision on why MS1 and MS3 need a 100k pull down resistor and MS2 needs a 50k one. But based on their design decision, my guess is it IS useful and that those resistors are just pulling the line low to ensure they are low unless the jumpers are enabled to set them high.