I just want to point out that there is no such thing as free electrical energy. If you are charging the batteries, you are putting more load on the engine which means that you need to burn more fuel. This is probably slightly more efficient than running a separate diesel generator but it isn't free.
Yes you are right in what concerns free energy but I disagree with the "slightly more efficient than running a separate diesel generator".
You could be right in terms of fuel consumption but in practice it is not slightly but much more efficient and I am not only referring to fuel, but also in what refers maintenance costs and space saving.
Even in what regards fuel efficiency you don't consider a very important point: If I don't have wind and I am motoring or if I am motorsailing, with that set-up, I am charging the batteries almost for free (just a very small increase in consumption) because with a generator, motorsailing or not with a regular engine, I will have to run the generator anyway at the end of the day (assuming a significant electrical consumption, with refrigerator and AC).
Also you don't consider that while sailing the spinning propeller will be charging the batteries. Of course you pay that with some speed, but if the boat is not very light and if the wind is not weak I suspect that would not matter much to most cruises, I mean some less speed.
It is not only the fuel for the generator, but the hours of running time (maintenance) and most of all the noise and the discomfort.
All in all this seems to be a great system for someone that cruises out of marinas and has a significant electrical consumption.