It's not quite as simple as it sounds as the main electric motor / petrol engine / secondary electric motor-generator are all mated together through a planetary gearbox. It is all very clever engineering and was a long time in development. So instant copying not possible, hence BMW only doing a seperate crawl home generator as a REX. The fully integrated voltec system provides opportunities for improving the high speed electric motor performance in EREV mode by partially using the engine torque to drop the revs of the main electric drive motor, so the most can be got out of each gallon of petrol. Having said that, Tesla have put a generation time limit on the need for this complexity. My Ampera is very probably the last car I will buy which I am happy with but the future does look bright EV-wise for younger folk.The Others... sounds Lost ;p
I have been pondering why this generator / battery approach wasn't done way earlier? Sounds great!
It's actually a lot better than that - but the Telegraph story seems to have lodged firmly in the public's mind....hence BMW only doing a seperate crawl home generator as a REX.