If the diode is open and will stay open you will not probably hurt much in the short run. Sometimes an open diode becomes a shorted diode depending on how the pieces got blown out on the rectifier plate. Sometimes they kind of move from open to shorted according to temperature or load.
If you have a blown diode on an alternator, it is unwise to continue to depend on the alternator.
I understand your question. Basically you want to know if the alternator will function well enough in the short run at only 2/3 capacity. That mostly depends on if the alternator will continue to function at 2/3 capacity and if you keep the electrical load light. No lights, no A/C, no radio, etc., etc.
