I can't see any reason because the neutral will normally not be carrying any current and thus no parallel current paths would form.
If two star connected sources are connected together as shown, then unless the currents in the lines are perfectly balanced there will be a balancing/residual/zero sequence current flowing between the star points. There will be then a voltage drop along the interconnector between the star points and consequently the two star points will not be at the same potential – there will be potential difference between them. If one star point is earthed no current will flow to earth. However, if the other star point is earthed as well there will be a current flow through the earth between the electrodes; the magnitude of this earth current depends on the relative conductance of the star interconnector and the terra firma path.
If an installation’s earth electrode is placed near one star’s earth electrode and it is connected to the Protective Earth(PE), in the presence of unbalanced loads there will be a potential difference between it and the star electrode. Once again, there will be a current flow in the circuit including the star electrode, section of interconnector to PE tap off, PE, installation electrode and terra firma between electrodes. If the other star is earthed then there will be two circuit paths. So, the first thing to note is that with neither star earthed, there are no terra firma circuit paths which includes the installation earth electrode so no current flow through the installation earth electrode. So by leaving the stars disconnected locally from terra firma, and then connecting the PE to the installation earth electrode, the local terra firma determines the potential of the PE not anything further away. This means the local terra firma around the installation earth electrode is substantially at the same potential – an equipotential zone.
The particular thing about the installation you have mentioned is that the supply is 3 phase 3 wire with no neutral. All loads are three phase or connected phase to phase. The PE has no current carrying function in the delivery of electrical power from sources to loads. The PE (PEN) is solely to ensure automatic disconnection of supply ( through the operation of an overcurrent device) in the event of a line to earth fault or line to metalwork by a low impedance connection to the star(s) of the sources (Zexternal) to achieve disconnection times. By creating a localized equipotential zone and connecting all extraneous metal work to the PE and at a number of points connecting the PE to terra firma within the EP zone, the touch voltage between terra firma and energized metal work is kept low during a fault condition. Even if the fault current is high, the volt drop along the PE(PEN) raises the remote stars in potential with respect to their local terra firma and not the potential of the installation’s earth electrode or anything connected to it. This is the safety benefit of anchoring the PE(PEN) at the installation alone. If the remote star(s) were earthed and there was no connection of the PE to terra firma at the installation, it would be the installation end rising in potential creating the possibility of dangerous touch voltages.
If the PE to the installation is broken, the equipotential zone is still preserved and anything within it connected to the PE will be at the same potential so no prospect of unsafe touch voltages between terra firma and metal work and between metal work earthed by the PE and a common point/points in a common zone. This applies too for a bolted line to PE fault in the installation at the same time as a broken incoming PE(PEN). ADS would happen if there was a second line to PE fault. But there would be no rise in the potential of the PE with respect to local terra firma.
So why is not one or both both stars earthed and a local earth of the PE the best solution? I hope you can see that this set up does not ensure the installation’s potential – that of the equipotential zone and that of the PE – stays below a safe touch voltage. (As an aside, the arrangement of one main earth electrode at the installation also reduces EMI through the avoidance of the earth loop currents I mentioned earlier.)
I don’t know how this set up is officially described. My thought is 3/3 TN-PE where 3/3 is 3 phase 3 wire no current carrying neutral; the I refers to the stars of the supply, the TN to the switchgear located connection to terra firma of the incoming N/PE/PEN and PE to what is connected to all metal work in the installation.
Or something along these lines. Practising power engineers can amplify or improve(correct) on what I have tried to explain to you in response to your thought-provoking question.
