We do not use GroundSpeed because wind does not matter when calculating Rate of climb. With ClimbGradient we measure the angle of climb which is relative to the air mass, NOT the angle of the aircrafts flightpath (geometric path relative to ground). With ROC we measure (ft/min), how high we get in every min of climb and as it is a function of the Climb Gradient it is also relative to the air mass.
3 words...
WTF...