Hi Bob - I think that might be an oversimplification of the benefits of a fast-responding, accurate asset on the grid. The underlying principle of why storage is beneficial is that it enables broader system optimization, not the round trip efficiency of any one charge.
Modulating large, slower-responding generating assets (like a natural gas plant) to perform ancillary services is inherently quite inefficient. These either need to be on and idling (burning fuel but not delivering electricity, or even running efficiently), or modulated at the turbine and jumping between different levels of efficiency. Also, taking minutes to respond to a grid signal means that the plant can be following a signal in the wrong direction, wasting energy and needing further correction.
By using storage systems, we can address two big inefficiencies in the system that will help reduce wasted energy. With storage allowing grid operators to more efficiently use thermal plants and deploy the ESS for grid services, we can have those fossil plants at peak efficiency more of the time. And, by avoiding building new combined cycle peaking plants and instead using storage to address those handful of hours each year, we can leverage clean energy or optimized fossil energy to address those rarer seasonal needs.
It does, as you noted, save utilities and consumers money, and part of that savings can be seen in reduced demand for frequency response on the PJM Interconnection following the introduction of pay-for-performance, as one example. Less energy bought for FR service because of the speed and accuracy of storage means less fossil fuel burned, and the energy that is burned is done so more efficiently.
Storage is a multifaceted asset on the grid, and the value exceeds any one application - and while it does lose power in its round trip, it enables us to use energy more efficiently, helps level out variable renewables and maximize their output, and defers unnecessary investments in new transmission equipment and power plants.
The grid is a large interconnected system and it is aging quickly and not very nimble, and looking at any one node or one energy transaction does not show it's wider impact and benefits. Storage is among the solutions (including efficiency, DR and renewables) we need to deploy to to enable the next generation, efficient electric grid.