Just an adder to Jim H's excellent history lesson.
The way the old two brush generators worked, all the current had to pass through the brushes. In the alternator, only the excitation current passes through the brushes, thus the brushes are smaller and more trouble free. Additionally the alternator provides a good bit of energy at engine idle speeds. The old two brush generators produced virtually no energy at idle speed.
Additionally, I don't know about their application in an automotive alternator, but I can never remember seeing a diode in other circuitry short. If they fail they typically burn open.
Have a great day,