The decreased propagation time through air vs glass likely wouldn't yield latency improvement in relevant orders of magnitude. The primary benefit is cost reduction. Erecting a few towers with optical relays over high points in terrain would be cheaper overall to install & maintain than either aerial or trenched fiber. If a 3rd party builds something in your way, just relocate the tower whose line of sight becomes obscured (on the assumption tower sites are solar-powered and self-standing).