It's a good question. I do have a 'charger (an ex-Toyota MR2 SC14) which is in the process of being bolted up to a Transit 205 Pinto. Sorting out pulleys and belt tension has been a pain, and the charger does suck a huge amount of power from the engine - so while it will be more powerful than NA, fuel consumption is going to be high.
I guess the main reason I'm doing it is just for the experience - go through the process, learn how to install a 'charger. There's a certain coolness in having your bonnet up at a show with a supercharger on show. Also this one has an electric clutch on the input shaft, so you can switch it in or out, so I have the option to either have a big red button on the dash, or to wire it into the ECU to come on automatically. But I've no good engineering reasons for doing it and turbo would have been better for power and ease of fitting.
My next engine is a T5 (ST-225) and it has a turbo - much simpler, and I wouldn't think to supercharge it instead.
I know it's widely held that turbos don't rob any power, but they must take some - obviously not as much as a supercharger, but it must be harder for the piston to force the exhaust gasses out if there's a turbine in the way, rather than an open pipe?