am I hurting anything by continuing to use my old cheap Radio Shack diode?
No, nothing is suffering at 13 VDC. Many avionics are rated at 10 to 30 volts. Alternators have failed many times.
The pilot keeps flying as the voltage slowly drops to 13, 12, maybe down to 11. The avionics are not destroyed.
There may be exceptions. But at 13 volts, everything should be happy except batteries that want 14 VDC or more to be charged.