I can see the alternator driven off the propshaft saving power on race cars but not in top gear cruising. revving hard in lower gears will have less drag/load from the alt due to its spinning slower than the motor in those gears, but in top gear cruising it would cancel out and voltage drop from the long wiring would also hurt.

u could save fuel slightly at idle from no load from the alternator, but after a long time the load would increase when driving to recharge (as dodgeman already noted), and the best way to save fuel when idling at lights is to switch off anyway.