1) Espionage is anything but cost-effective. But cost isn't the primary (or even secondary) concern there for those who want to do the spying. (It's (technical) feasibility)
2) Running cable above ground is _always_ more cost-effective then running cable underground. So if you:
- don't give a shit about your customers
- don't have a lot of competition because you can gain a monopoly by buying senators
- and if you do a bare minimum of maintenance because you want more money (more so if you _do_ run cables underground)
then even in a city, local power stability is going to be shit.