Sure,
some back pressure is necessary. But it is an engineered system; it's not a "If some is good, more is better" deal. Rather, the exhaust system is designed to a local optimum.
Let's think about the power requirements of the alternator for a moment. Let's assume it generates 50A at 14V. That's 700 watts of power. Assuming an total efficiency of 70% (that's unrealistically high, especially for turbo!), you'd need to extract 1000 watts of power from the exhaust, or about 1.3hP. Again, "There Ain't No Such Thing As A Free Lunch." The exhaust gas pressure simply isn't high enough to extract that kind of power. An alternative way to think about it: how would your car behave if you stuck a 1/2" diameter exhaust pipe on it, all while leaving the catalytic converter and muffler in? Better yet,
here's a real example of a what happens when you restrict the exhaust.