GPS groundspeed is indeed useful for aviating. You are straight and
level in IMC flying along into a 20 knot headwind at full power. Your
indicated airspeed is 70knots (80 knots true at your altitude and
pressure etc). You notice the groundspeed is 80 knots. All is well. You
get ice, and note your indicated airspeed has dropped to 50 knots. But
you note your groundspeed is 75 knots. Are you on the verge of a stall
that 50 knots indicated would tell you? Or are you still flying as
close to normal cruise?