It isn't clear (at least they didn't go into any detail) WHY maintaining the mains frequency is more costly than letting it drift. When you are feeding power into the grid you have no choice but to do it synchronously or else blow yourself up.
The frequency of the grid is a product of the rotation of the generators feeding it. While, yes, if one generator tries to drift it will be 'forced' to stay in sync with the grid or bad things happen. But the overall frequency can drift faster or slower then a perfect 60Hz
In order to correct the whole grid, a power station has to force it's generators to run (slightly) slower or faster then the grid. This creates a power flow between the correcting station and all the other stations as the correcting station station is 'pushing' on the other station's generators to go a little faster or slower.
This requires a impressive amount of power to do that is wasted on just slightly changing grid frequency. If they could do this left often == savings.