I think the following post I found over in the F3HR forums does a good job summarizing why it doesn't matter for running...
Even for hiking, the efficacy is probably dubious due to trees and what not already bastardizing your GPS signal.
If I think about hiking, I think about triangle, then I think of the Cosines. 5 degree: horizontal distance is 99% of the real distance. 10 degree: horizontal distance is 98% of the real distance. 15 degree: horizontal distance is 96% of the real distance. 20 degree: horizontal distance is 93% of the real distance. 25 degree: horizontal distance is 90% of the real distance. 30 degree: horizontal distance is 87% of the real distance.
With 3D distance OFF, which distance is measured? The horizontal or the real distance? And while GPS signal might be compromised by trees, in a stabile weather, is barometric altimeter accurate enough to measure the elevation change?