"Spin down" also lowers the power line frequency, from 60, to 50 hz to 40 hz and the lower frequencies, as they approach DC, start causing large current surges through wound coils.
I suppose that'd be true if the voltage didn't diminish at the same rate as the frequency. I think it does. So, I'm not convinced there'd be large current surges...
And even if there were surges of some degree, I can't imagine what coils (transformers, motors, etc) would be destroyed by a 'spin-down' surge lasting a few seconds.
Yes, I've also read that leaving a load on a generator as it spins down can destroy residual magnetism, requiring a re-flash. I guess I'm inclined to see value in an under-freq or under-voltage lock out - mainly to disconnect the load if the engine can't maintain 60Hz for extended periods. THAT can certainly cook induction motors, and transformers. But I don't see how 'spin-down' is going to kill anything other than residual magnetism...