RRR already posted one of the big reasons that electric fans started to appear, transverse engines. But now you can pretty much find them on everything, even some semi's are running them now. The reason for this isn't cost, at least, not the cost of the fan system. A mechanical fan doesn't need all the electrical components and systems that an electric fan does, bolt it on and go. With an electric fan set up you need relays, motors, switches, feet of wiring, a separate circuit, etc. But what you gain in all of that is precise temperature control, and that's why they're used on most new cars. The ECU controls when the fans are running, so, you can keep the engine at it's ideal operating temperature more often and more reliably. And that's efficiency, and that's gas mileage.
As far as efficiency, it's not just the conversion of energy you have to consider. Yes, an electric fan requires energy to be converted from mechanical to electrical and back to mechanical, and there's loss there. But, every second a mechanical fan is spinning that an electric one isn't is lost energy. If you've got a thermostatic fluid coupler on your mechanical fan that won't be a huge amount of time, but it will still spin more because of the way those operate, it takes time for the temperature change to register. And then there's the efficiency of the fan blade itself. A mechanical fan blade has to work over a wide range of RPM's, anywhere from idle to full throttle. Now, if you have a thermostatic fluid coupler it won't always be spinning 1:1, but it will still operate over a wide range of rpm's. And over that range of rpm's the fan blade has it's own range of efficiency and a wide range of CFM output. A fan blade designed to spin at 3k rpm will be most efficient at that speed. Sure, it will still move air at 5k rpm and at 1k rpm, but it's not the most efficient there, you're losing energy. An electric fan has a significant advantage there, it always spins at the same speed. So, the fan can be designed for peak efficiency at that speed and it will always be there. Unless of course you have a 2 speed electric fan, but in that case the fan is still designed for the lower speed and the high speed trades peak efficiency for CFM knowing that the high speed isn't where the fan spends most of it's time.
With an electric fan the better the controller the better the fan will work. New cars are controlled by the ECU, so there's a tremendous amount of control there, it doesn't just have to be a temperature range like the fans come on at 200* and turn off at 205*, the ECU can turn them on and off based on the temperature of the engine, engine RPM, speed of the vehicle, output of the oxygen sensor, you name it, they can be as complicated as you want. And that's where the retro-fit electric fans usually have their problems and why people say they don't work, they hook an improperly sized fan to a cheap generic temperature switch and are surprised when the result doesn't cool well. Same for reliability. I use an OE electric fan set up from a Ford Contour. It was designed and tested by Ford for use on a car what was designed to last hundreds of thousands of miles. Aftermarket fans, even the high end ones, don't necessarily get that kind of testing.
A good electric fan system is going to cost more than a thermostatic fan clutch, mechanical fan and shroud. Possibly a lot more depending on how you do it. You'll need a fan that can push a few thousand CFM, at least one relay, a good fan controller (not just a cheapo temp switch sender), a bunch of heavy gauge wire and probably even a higher output alternator to power a fan that pushes enough CFM. With all of that done properly, the electric fan should be more efficient and do a better job of keeping the engine within it's operational temperature range. I know mine does. My "new" '74 Duster- or why I need a project like a hole in the head
But there's a lot of work that has to be done to make that happen, and since none of that is on the car already it's probably better to spend the money on a good thermostatic fan coupler and a shroud. It will look stock and should be up to the task of cooling all but the really high horsepower cars.
