It may seen like that but it is the only way to detect a phase to ground leak/fault in the 'isolated' system.
If you did not have that ground connection then no current would flow into the ground fault and you would not be able to detect it. With this circuit you will have no current in the earth point if you have a balanced load (and supply).
If you have a fully isolated system you cannot detect a faulty to ground from any phase (or neutral) as no current flows. All that happens is that your system gains a real ground reference at some point (perhaps not the best point) but no ground/fault current will flow until some point of your system connects to a point with a potential difference, this will cause a current to flow.
Having no way to detect the single fault is risky especially with high voltages or current. If your circuit must be floating it could float at any voltage and possibly exceed insulation capabilities.
On consideration I think it may be the best way to manage the 'isolation' and it will reference your circuit to ground at the virtual neutral point. If your circuit must be isolated so it can float to the load potential then you should reference the ground to a suitable neutral point there.