From the powerline point of view! I can think of a number of issues.
Whilst rain water and to some degree tap water may not be an issue, water from other sources could contain enough contaminates (possibly even the retardant chemicals) to cause a serious electrical arc onto the ground/vegitation if a large amount is dropped on a live powerline.
Similarly a few tonnes of water dropping on a line could cause structural failure, bringing the line down onto the ground similarly causing arching.
Neither of these would, I think, help in preventing the spread of fire and may even cause secondary ignition of vegitation.