It still matters: Try engineering a high capacity (350 to 1000 Mbps) PTP microwave link at a distance of 80 km between two mountaintops... Path loss and channel capacity are real issues to deal with even at distances much shorter than GEO.
Again, not my field of expertise, but worth asking: There's going to be much much more air between two mountains 80km apart than between the surface and 80km altitude. I believe the quantity of air in a column above you at sea level is about equal to 9km of sea-level air. How would that change the problem?
