> This is a mild curiosity at best.

Exactly. There are genuine economic/engineering reasons for negative prices to occasionally exist. But in a well-designed, well-run, grid price will be negative only a small minority of the time. It just doesn't make sense to install a bunch of expensive equipment to provide this service when sufficient capacity exists from "happy accidents" like spare battery storage.

In the long run, better managed solar and wind should make negative prices a fairly rare event.