That would be an extreme psychological multiplier in such a prolonged attack. If the repair crews could not be freely dispatched to damaged locations for fear of snipers waiting for them when they arrive, then it would grind repair work to a halt. A simple cut pole in the middle of a field would turn into a potential kill zone for a utility crew that came to fix it. The attackers would own the night.
In the book, he killed the lineman when he was up the pole so his body was hanging there when others arrived to see why he hadn’t checked in with them once he saw the problem.
The sniper would never be found since he could be over 100 yards away, plus the new arrivals wouldn’t know if the person who killed the lineman was still there.