Re: communicate with the grid... mmmmm
I am talking from the point of view that I worked in the power industry for a number of years with direct experience of the issues.
Ah the "resort to authority" approach. Pity you don't seem to have learned all that much in that 40 years then.
The assertion 'frequency has to be, give or take near-unmeasurable differences' the same across the grid is just nonsense, for a start it defies the laws of physics!
It defies the laws of physics for the whole grid (talking UK, all AC connected, grid here) to not be at the same frequency. There may be phase differences, but over a few cycles these cannot be more than an imperceptible difference in frequency. Lets face it, if the phase different across the grid exceeded some "not very large" fraction of a cycle, then things would be tripping or going bang due to the excess currents caused.
Just see what happens if you try over-driving a generator. It'll change phase (become more leading as the power goes up) - but it will not change frequency as it's locked to the grid. Only if you drive it so hard that it electrically "cannae do no more" then it will slip - yes it'll now be running at a different frequency, but not for long before either all the breakers trip or something goes bang in a big and spectacular way !
So some small fraction of a cycle, averaged over (at least) a few tens of cycles is going to be "fook all" in terms of frequency deviation. At very most you could get a small deviation to register if (for example) you suddenly opened the taps on a big generator and significantly changed the current flows in part of the network. In the long term (where long is measured in seconds), phase differences across the network due to power flows etc simply cannot show up as a frequency difference.
The US, with their larger grid, for precisely this frequency control problem, section their grid using Direct Current inter-connectors, but I digress.
You are confusing the terms here. The US grid is decoupled for stability control, not frequency control - though the two are fairly closely linked. It's due to the increased difficulty of the problem of balancing supply and demand across the much larger number of organisations involved.
Simple example, of a large generator in (say) New York trips, then that will have an effect across the whole network - reduced voltage and reduced frequency. If the whole network were AC coupled then you could then find generators in California opening the taps to compensate - when what you really need is generators local to New York to open the taps. The result could be significant changes in power flows, and if load is high then the risk is that you exceed capacity on some line somewhere with the risk of that tripping and causing a cascade failure (cf Niagara Falls incident).
Yes, the phase across the network will change, but the relative frequency (once you average ofer a few seconds) will still be exactly the same.
And this is why we have A (note the singular) central control room orchestrating our grid. We don't leave it to all and sundry to try doing their own thing, the control room orchestrates it - just like an orchestra would make a horrible cacophony if all the musicians tried to do their own thing rather than have the conductor organise them.
Splitting the US network with DC interconnects allows frequency control to be done in several smaller regions - thus simplifying the task somewhat. In addition, over long distances (which they have in the US), HVDC can have lower losses than AC.
It's also the same reason all our inter-country interconnects are DC - it avoids all the problems that would be caused if we had to co-ordinate our frequency control with the rest of Europe. Not to mention, the AC-DC and DC-AC converters offer very easy power control, much easier and quicker than tap changing on a transformer.