I also have never been told a scientific reason why not to bond the X0 when a Y is used as the primary side of a transformer, the closest I have been told was that when using a common 3 legged core transformer eddy currents will build up and over load the X0 and the grounding conductor and cause the transformer to over heat if the X0 is bonded back to a Y supply, this is if the secondary is unbalanced.
I have seen it happen many times just never told the real reason why? at least scientifically
Wayne A. From: N.W.Indiana
Be Fair, Be Safe
Just don't be fairly safe