Edit: to answer my own question, they look pretty good [1]. Significantly lower storage density than batteries (roughly 50Wh/L versus 420Wh/L for Li-ion), but still dense enough for this to be workable at grid level. The linked presentation proposes converting decommissioned power plants into grid-level capacitor storage facilities, since the transmission switchyards are often intact. Furthermore, all the technology is available today, and when built, capacitors require almost zero maintenance.
[1]: https://www.energy.gov/sites/prod/files/piprod/documents/Ses...
For technical reasons, thats because the 'spinning reserve' - which is momentum of every synchronous motor and generator across the nation - already adequately handles this.
Also, large generation stations are required to have a 'load line' which damps high frequency oscillations. The load line can best be described as 'whenever the grid frequency starts slowing down, generators must put more energy in automatically'. Things like wind and solar typically don't have the ability to do that.
For human reasons, it's because electricity markets tend to be minute by minute at most.
For all those reasons, I don't think you'll make any money with capacitor energy storage banks (not to be confused with capacitors for grid scale power factor correction, which can be profitable).
The problem is that capacitors (assuming you mean super and ultra capacitors) don't have much capacity compared to batteries, so their cost per MWH of storage is high even if their cost per MW is low.
If one were to build a "facility" to hold short term load, that's likely what they'd use. They just can't be used in cars because they're incredibly volatile and don't hold a charge for a long period of time.