Coordinating the power grid needs information, today the information carried for proper coordination ( AC waveform detecion by grid inverter ) is not sufficient to make things work at large scale.
Whether you transmit the coordination signals ( i.e. extra bits of information ) in-band ( over powerline, superimposed on the AC wave ) or out of band on a separate carrier is the question of implementation. Both in-band and out of band would have their upsides and downsides.
Fact of the matter is that todays grid is not built for that.
Simple case, assume there is a dozen or so high power solar installations in my neighborhood, delivering most of the peak power output. The requirement for grid-tie inverters is matching the phase within 1% of the 50/60hz waveform read at the connection point. The question is, _whose_ waveform is mine really following ? Is it the neighbors ? Which one is his following ? How does the error propagate and does it multiply ? That is the gist of the coordination problem.