I'd say the instant electric showers have become very popular, particularly for add-ons to existing properties. As you say, the fact that the plumbing needs only a single cold pipe run from any convenient mains-pressure line makes them an attractive option where minimal damage to existing decorations is a consideration.
Even in some major remodels, a lot of people these days seem to prefer these to a fully plumbed non-electric version. Where hot and cold are fed to a conventional shower from the low-pressure cistern in the attic, the cold feed is generally run as an entirely separate pipe from the cistern. If it were tapped off some other low-pressure cold pipe there would be the danger of someone turning on a tap elsewhere in the house and causing the shower temperature to rise suddenly.
I'm sure that the complexity of adding all the extra pipework has contributed to the popularity of the instant showers, especially for DIY installations (Hot topic at the moment, but the DIY mess that arises from some people trying to do the wiring on these high-current units is another story!).
Re the cost of running them, a 9.6kW unit used during normal daytime rates in the the U.K. would cost about 70 pence per hour, including tax.
