[nl]

Of course we care about the energy amount stored - some. But total power capacity is more important for a large number of applications.

The usage the OP listed (arbitrage) is only one of the 9 function listed by the National Renewable Energy Lab[1]. Many of the functions need immediate power capacity in the seconds to an hour (so MW seconds or MW minutes), and more important is the speed of response and the amount of power deliverable.

[1] https://www.nrel.gov/docs/fy19osti/74426.pdf see table 1

[jupp0r]

You are describing perfect use cases for large capacitors, which was my point earlier.